View Full Version : Bad fuel economy ???
I have a 96 C3500 crew cab dually, 116,000 miles, 6.5TD, 4 speed auto, 11' Lance camper. The rig tips the scale at 10,500#.
DRIVING CONSERVATIVELY, I average 7-8 mpg, while a friend with a 16,000# motorhome averages 12-14 mpg (same engine and under the same driving conditions, but 50% more weight than mine). Does this make sense or is there something seriously wrong with my rig???
I'd especially like to hear from people that have a rig like mine, to know what kind of mileage they are getting. My engine is turning about 2500 RPM at 65 mph in the highest gear.
Bill, I have '93 extended cab dually with 6.5TD, 4L80E trans, 4.10 rear. 80k on truck, 20k on engine. I too have 11' Lance camper. Combined truck/camper empty weight is 10,060# Most of my miles are interstate at about 65mph turning approx 2450rpm. I also frequently tow a Jeep wrangler, another 3500#. With the Jeep in tow, I get between 10 and 11 mpg. Without the Jeep, I get 11 1/2 to 12+ mpg. Without the camper on, I get 17+ mpg. I'm happy with that. Hope this helps you.
Bob
richard7
11-02-2003, 09:15
Coupled to 4l80E, and 4.10 diff. ratio, turning 215/85/16 rubber (same diameter as 245/75/16), the engine should turn 2200 RPM at 65 MPH, not 2500.
If this is your situation, look at a defective torque converter lock-up clutch operation. A non-operating TCC will increase fuel consumption significantly.
Richard, With 4L80E, 4.10 and 225-75R-16 tires with a diameter of 27.5 inches, my 2450 rpm at 65 mph is right on the money. I used the excel rpm chart that was available here on the DP.
Bob
richard7
11-03-2003, 06:54
OK. But the thing that doesn`t match is the tire diameter. 225/75/16 is supposed to be 29.3 (245/75/16 is 30.5), leading to a 4% increase in rpm, ending at 2350 @ 65 MPH.
Bill-
Mild correction on the P3 motorhome mileage:
11.5 combined (but for a motorhome that will incude lotsa highway, w/a fair component of hills, like Donner Summit, Grapevine, Tehachapi...)
14.0 on the flat (like I5 between Redding & Grapevine).
My engine has soot trap only, no CDR, which is an advantage, but not 50%, so I'd still expect bettern 8 w/the Lance.
tom.mcinerney
11-03-2003, 18:27
2500 rpms too high for mileage. above comments (diff ratio, tires, TCC lockup) good. low turbo boost will also decrease mileage.
Yes, you definately don't want the wastegate dumping boost. I would not be surprised if you have a problem with the boost control system. Check for full vacuum at the wastegate actuator. It should be at least 20" or better. If not then you could have a bad vacuum pump, bad wastegate solenoid, or leak in the lines.
Hi all, I don't know what size tires Bill has, but for my 225/75R/16s, 2450rpm at 65mph is right on the money. Here's why. I agree with you Richard that the chart says a 225/75R/16 is about 29.3" in diameter. However, that's before you put any weight on it. My weighted diameter (top of tire to ground)is more like 28.75 inches.
More importantly, the weighted radius is 13.75 inches(center of axle to ground). This weighted radius is what determines what the working circumference is because the tire is going to run with that big flat spot on the bottom all the time.Therefore, 2 x radius x pi (2 x 13.75 x 3.1416) = 27.50 x 3.1416 = 86.394 inches divided by 12(inches to feet) = 7.1995 feet circumference.Then, 5,280 (feet in mile) divided by 7.1995 (circumference of tire in feet) = 733.38 revolutions per mile. What you have to take into account is the "loaded wheel radius". If you are running empty, it will most likely be slightly greater than if you are heavily weighted down. The above figures for my vehicle are with the camper on (10,000# plus gross). Even at 65mph, the centrifugal force generated by the tire spinning approximately 794 times per minute won't appreciably affect the footprint. Cheers.
Bob
richard7
11-04-2003, 06:32
Right again. But the odometer doesn't know if the tire is loaded or not, so it will read more MPH than real for the same RPM. 2500 is high.
Bob T & Richard7,
While you two guys bicker about whether I'm driving on flat tires or whether they're just flat on the bottom, I'm sitting here on the bottom line. Namely I've got the same size tires (LT245/75R16) as Bob T, and the same camper with the same engine, same total weight. What else is there? I see Bob T has invested in a few aftermarket goodies. Could that be the difference between 8mpg and 12mpg?
I'll do the vacuum test on the wastegate valve today and see if that's up to spec.
Bob T's, few aftermarket goodies are pretty extensive & expensive. They are probably making some of the diff. plus he will probably leave you in the dust when side by side. He is probably running twice the boost than you and a little tweak of the pump. If I were guessing.
When I ran stk wBanks exhaust & 10psi I was around 10-11mpg loaded. I think you should run better than 8mpg. The thing I have noticed is my camper has the biggest effect on mpg.(wind resistence) I get about same mpg w/camper as I do w/camper and trailer. Maybe I lose 0.5mpg. I run C2500 4L80E 4:11 rearend 245/75/16.5 w/camper,car trailer and FJ40,total wt truck & stuff 13,000lbs I get 12mpg running +-70 mph(2500rpm maybe 2550)up & down the hills of KY and have plenty of power to boot. For the I/C to be effective boost should be over 12psi.
Empty ranges from 15.5-17mpg depending how
I drive it. It's just real fun to drive now.
[ 11-04-2003, 12:44 PM: Message edited by: cruzer ]
My vacuum was 21" on the wastegate valve, so I guess that's not the prob. The tow-truck driver came by with his '99 F450 diesel tow truck to talk about hauling my truck up to the Chevy dealer for warrentee IP/FSD replacement. He said his tow company has several diesel F450's with 300,000 -400,000 miles which average 10mpg with misc injector replacements, etc. but no major rebuilds (I imagine working hard). He said most tow companies use Ford and some Dodge, but no Chevy. Hmmm... wonder why??? could it be reliability???
The tow driver said he has a friend with the same Chev truck and camper as mine, but his friend has the big gas engine, getting 10mpg. He said the stock 6.5TD was underpowered for my dually/crew-cab/camper. I recently "hit the wall" at 65mph with a headwind on the freeway and am inclined to agree. I love you guys on TDP, and I rolled over and spread my legs for the sexy Chevy image that GM jammed down my throat on TV, but unless I can figure this out, I'm in the market for a Ford or Dodge diesel as soon as I can dump this POS Chev. I'm just not prepared to throw several thousand bucks into aftermarket mods to get this rig back up to Ford or Dodge standards.
Bill The extra goodies I added are for engine health/longevity and more power which they do provide. However, I don't think they account for our differences in mpg because I did much better than you in the mpg department when my truck/camper setup was stock. I almost always got 10 mpg+ in stock truck/camper setup towing Jeep on the interstate highways(mostly between Tucson and western Pennsylvania) unless I was pushing a moderate or greater headwind. With the extra goodies, I do much better now against headwinds both in power and mpg. My '93 is mechanical injection pump; your '96 is electronic. Maybe the low mpg has something to do with excessive fuel flow rate in the electronic injection pump caused either by faulty pump or computer. There are experts here on the DP that understand the workings of the electronic injection pump and computer and can give you advice in this area.
Bob
I don't know what the odometer knows. What I am expressing is that if the Excel - rpm program is accurate, then a tire with a working diameter of 27.5" coupled with a 4.10 rear end and 4L80E trans in OD will at 2443rpm, produce a vehicle speed of 65 mph. That is what the chart states. I know by measurement that my working tire diameter is 27.5". When I look at my odometer while driving down the highway and it is reading 65mph, my tach is reading approximately 2450. Therefore, I think my gauges are fairly accurate. I also think that Bill H's 2500rpm at 65mph would be in the ballpark (allowing for variations in tach or odometer accuracy) if his tires were the same diameter as mine. Since he is running 245/75R/16s, his diameter is greater and 2500rpm seems a little high (by150-200rpm) for 65mph.
Bob
richard7
11-04-2003, 22:39
1.- I would have a look at the exhaust system. The OEM exhaust system is quite restrictive, and if you still have the catalytic converter, it may be aggravating the restriction as it plugs over time. If it is plugged, it will decrease MPG and engine power a lot, as the engine only try to push exhaust gazes outside. You will get some black smoke too.
The exhaust is the first thing to improve, and it applies more on your application as it gives greater results on a high power demand application. You can get a bolt on low restriction system for less than 500$.
2.- Try to check your boost pressure. Low boost would mean incomplete combustion and poor fuel economy. (does it smoke black?)
3.- Does your lock-up work in OD. If you are running 245/75/16 tires as you said (unlike BOBT 225/75), your RPM is too high. But it may be that your tachometer doesn't read correctly.
4.- '96 powerstroke or Cummins are not much more powerfull than the 6.5, but they are medium duty engines and the 6.5 is light duty. So they have an higher average life expectancy. You can have to put money on any used engine you are buying.
5.- You are not obliged to beleive the 10 mpg 454 big block...
Bill H,
Just for reference sounds like we have identicle trucks. I get 12.5 mpg with normal city driving on a tank of fuel. This includes approximately 1-2 hours of idling per tank. I get the same mileage pulling a car/car trailer around 4k. HWY mpg's are around 16-17 empty. I'm pretty much stock with the exception of air box mods and K&N filter. 200k on the truck. Replaced injectors about 6k ago and noticed about 1-2 mpg increase. Just a little comparison for you.
I think you will find that the "loaded diameter" of the tire has little to do with the calculation. A while back I did some experimentation to determine the effect of tire pressure on rolling circumference and the conclusion I came to was there was no effect within the useful pressure range. I went from 65 psi down to 10 psi and there was no measurable decrease in the number of inches traveled per rotation of the tire. (Fact is I measured a slight increase!) I theorize that this is because the tread of the tire works more like a belt and prevents the circumference from changing in proportion to the change in "working radius".
Additional supporting evidence: I have 265/75-16's and a calibrated speedometer. Using tach readings from the Tech II, my tire diameter comes out exactly as calculated (31.65").
Marty Lau
11-06-2003, 13:49
Bill;
Another thought is it maybe time to replace fuel injectors. Recomended to replace injectors between 75,000 & 100,000 miles. Indications of poor injectors is excessive smoke at startup and poor fuel economy.
With 245-75-16 tires and running 2500rpm at 65 your ratio computes to a 4.56 or your OD is not locking up as some one else has suggested. So that would also be part of your answer to low MPG. The 4.56 ratio would drop you MPG by about 11% so if you need injectors and if that is costing you 2 mpg and the 4.56 is costing you another 1+mpg so add 3 mpg to 9 mpg it gets you to 12 mpg.
Just a thought.
[ 11-06-2003, 03:15 PM: Message edited by: 16ga SxS ]
Thanks for all ideas. I still don't know what to do next.
1. I never get black smoke but when I start cold I get a big cloud of white smoke for a second or two, then no smoke.
2. It doesn't make sense that if my tires are bigger than Bob T's, that my rpm is higher at the same speed. My rpm should be lower at the same speed. I know I'm going into 4th gear. There must be one rpm sensor for the drive shaft that gets calibrated to read mph and miles traveled, and a second rpm sensor that gets calibrated to read engine rpm. Can I do these calibrations myself?
3. How do I know what to do next? Cost of calibrating speedo/tach? Cost of air box mods? Cost of new injectors? I don't think I'll invest in a new exhaust system ($500 + labor?)until I've exhausted every other less expensive option, and even then, how long would it take to pay for the system even if I get an increase of 2-3 mpg. I'm driving to Phoenix (800 miles) for Thanksgiving and I hope I can do something before then to save on the fuel cost for the trip.
richard7
11-06-2003, 17:21
1.- Make sure what is your rear end ratio. A 4.56, as was said, is not a highway speed ratio, and you won't get very good economy numbers with this ratio.
2.- Make sure the lock-up is working. Going to 4th is one thing, having the lock-up engaged is another. To check it, you can go at highway speed in 4th, than press on the pedal to accelerate without a downshift. The engine rpm will readily increase something like 200 RPM without a vehicule speed increase if the lock-up is not engaged. If the lock-up is engaged, the engine speed will follow the vehicule speed.
3.- Get a new high flow exhaust system. Best thing for the money for your high load application. Heath diesel sell a complete system for your rig for 379$, shipping included (heathdiesel.com). Easy to install. Don't look only on the money return, but the engine exhaust temp. will lower, and you will get some needed more power AND fuel economy
My suggestions, hope this help.
Bill H,
What to do next??? Well, airbox mods are free! Check out Kennedy's website...should be under tech/tips. Injectors will run you about 300 bucks. Having read everything I would really make sure you are running 4.10's. If you're not smoking real bad I would bet you have 4.56's in the rear-end. First things first however, definitely make sure your TC is locking up in OD.
Phil Holmen
11-06-2003, 19:46
An easier way to test if you torque is locking up as suggested to me by another member is while driving at about 55-60mph with your foot on the gas, tap the brake with your other foot, should notice 200-300rpm increase...
Bill H. You can check your speedo by running down the highway take an odometer reading at a mile mile marker then run 10 miles by mile markers and take your odometer reading. You can live w/the speedo being off and adjust by math to compute actual mpg. That's what I do, mines about 2% off. As for adjusting the odometer, there is an in depth article in the "1999 feature articles
& product reviews" that is offered by TDP, that tells you how to do it. There is alot of other useful info in that book too.
If a complete exhuast is out of the question you can purchase just the down pipe ,which is the most restrictive part of the stk exhaust. Really the 1st things you should get are boost and Pyrometer(EGT)gauges. These gauges will help you tell what's wrong & will assit in tuning.
Marty Lau
11-07-2003, 09:34
Bill;
It depends on how long you plan on keeping your truck. If you plan on running it another 150,000 miles plus I'd consider some changes. If you do nothing else on the exhaust change the down pipe as it is the bigest problem and they can be had for about $125, and I also would check and see if you have an exhaust shop locally who can do a 3 inch system from a new down pipe back. Or some guys have had take off Duramax exhaust put on. The exhaust is a good payback on power and ecconmy. Unless you plan on doing a lot of power mod's 3 inch exhaust will do the job. Also sdo a search on past post on exhausts there was a thread for inexpense 3 inch exhaustes some where mail order. Injectors run about $40 each there are 2 sets of of NEW Bosch injectors right now on Ebay. The Injector change kit is about $30.
The other thing I would consider doing if it was my truck would be to go to a 3.73 ratio. That should give you enough power to run with your camper and better MPG.
If it was my truck and I was going to run it for another 150,000 miles I would do the mod's in this order.
1. Airbox mod's
2. Exhaust upgrade
3. Injectors
4. Gear Ratio change
You should get paid back for the mods over 75,000 to 100,000 miles. Your engine will last longer too with lower RPM's.
16gsSXS forgot the turbo master. IIMO the best
$70 you can spend. You will have to deal w/the computer, but I know other people have done it.
Thanks to everyone for all comments and advice. I did some mods and testing and inspections and measuring and research and now I've got even more questions.
1. I did all the air box mods per Kennedy's web site, except for tapering the ID on the upper intake casting. While I was doing it, being the eternal skeptic, I really did wonder who came up with this mod and if anyone has actually done testing to show it helps. I admit it all seemed to make sense, but I still wonder if anyone has proved it helps??? Why did GM put that cone in the box upstream of the filter element? So we'd have less power and get worse mileage? Every time I take the filter element out I see leaves and pine needles and dragon fly carcasses in the box on the upstream side. I suspect they've been stuck somewhere deep in the fender since before I bought the truck, and slowly get sucked up into the airbox. Is there an easy way to clean all the debris out of the fender?
2. I found the Excel RPM sheet on TDP and found it to be very cool. I'm inclined to believe the tire-is-more-like-a-belt theory than a variable-diameter-by-load theory. I rechecked my RPM/MPH data driving on the freeway today and came up with some revised numbers - 2400 RPM at 65 MPH and 2500 RPM at 70 MPH. My eyes aren't too good at the dashboard distance, but that's as close as I could read the gages and drive at the same time. The RPM sheet shows a difference of about 200 RPM between 65 and 70 MPH and my readings were only a difference of 100 RPM, so giving a little on the low side and taking a little on the high side, I came up with a differential gearing of about 4.4. Of course I still don't know if my tach or speedo or both are out of calibration. And this is assuming my tranny final gearing is .750. I always assumed my truck was all stock but maybe not. Is a 4.4 diff ratio possible? How do I know exactly what my ratio is? Is it stamped on the housing?
3. I wanted to check my speedo with mile markers on the road but didn't remember seeing any around here, so I googled "california mile markers" to try and find some nearby. I appears as if Calif is the only state that doesn't have mile markers, (calif must have had the forethought to set the funds aside for something more important like recalling Schwartzenegger, in case he starts groping again). I did both tests to see if my torque conver lockup was working and when I punched the throttle at 65 MPH, the MPH and RPM slowly increased in unison. When I tapped the brakes at 65 MPH the RPM didn't change at all. I guess that mean the lockup is working. But I did have a question on how the lockup is supposed to work. Is it basically there to prevent unneccessary downshifting when the throttle is punched? If it's just a kickdown preventer, the RPM would shoot up much more than 200 RPM if it's not working, right? What physically does the lockup do to save fuel? It's not really an overdrive or fifth gear is it?
4. I crawled under my truck with a vernier caliper, and in round numbers, my crossover pipe is 2 1/4", my downpipe to cat. conv. is 3", and my cat. conv. to muffler and out to the tailpipe is 2 3/4". Does that mean the previous owners have upgraded the exhaust system? Since Calif doesn't require smog testing on diesels, can I take the cat. conv. out and replace it with a straight pipe? Would that help my fuel economy?
5. I have an EGT gage from an ultralight aircraft. It was made to be mounted by drilling a 3/16" hole in the exhaust pipe within a few inches of the manifold, inserting the thermocouple probe and clamping/sealing the hole with a hose clamp. Would this gage work on my diesel? How would I know what temperatures to look for? Would the distance from the manifold to the thermocouple be critical?
6. I know the vacuum at my wastegate solenoid is 21". How do I know if my wastegate valve is working? How do I check my boost pressure?
7. What does a cloud of white smoke mean at cold start up? Otherwise the engine doesn't smoke at all.
8. Who all has a Turbo Master and will testify it could be the best $70 I ever spent? Is it hard to install?
The lock up clutch saves fuel by eliminating torque converter slippage. Depending on a couple of things, it'll engage at light loads between 45 and 55 mph, later under heavier loads. It only engages in 3 (drive) or OD. If it's working, tapping the break pedal SHOULD cause RPM to increase as the brake switch unlocks the clutch and the slippage comes back. Another way to check is to count the shifts under light load from a stop. You should get 1-2, 2-3, 3-OD then lockup. No lockup will raise your RPMs by about 200.
Have you verified the accuracy of the tach? Often when changing the alternator, folks end up with the wrong drive pulley, which throws the tach off as it is driven off the alternator. Confirm this, but I recall the pulley diameter should be about 2 9/16". Your average Pep Boys rebuild comes with the pulley for a gas engine.
There is no 4.4 axle ratio. 3.52, 3.73 (standard), 4.10 and 4.56, which is very rare. The ratio it came from the factory can be determined by the RPO codes in the glovebox. There's a table on the DP that explains what they mean. You can verify the ratio by rolling the vehicle forward (or backward) 1 revolution of the tires and counting driveshaft revolutions. 3 3/4 is a 3.73, 4+ is a 4.10 and 4 1/2 is a 4.56.
Check right behind the passenger's side bumper tip for a hole leading into the fender. Early trucks with the snorkle were vacuuming the roads through that hole. Later trucks have no snorkle and the hole is (partially) plugged by a flap of rubber material.
richard7
11-10-2003, 08:54
As JC said, you should have seen a RPM increase when touching the brake, checking for lock-up operation. Regarding my suggestion of pressing the fuel pedal at highway speed, 65 MPH is probably too high in your case, because the engine speed is so high (2500) that you are inside the stall speed of the torque converter. So the RPM change is less perceptible that if you go at a lower engine speed. If you try it again, do it at a lower speed like 50 MPH or so, but you have to be in 4th.
You have a GM, OEM exhaust, because the pipes are 2 3/4" OD (GM used this size for many years).
White smoke when starting is a sign of incomplete combustion (cold combustion) of some cylinders. In low temperatures, it can be consider normal to a certain point, but otherwise, you may have to replace the glow plugs or check the system.
When I tap the brakes at 65 MPH or even 50 MPH the RPM does not change. The thing I have trouble understanding is that at 50 MPH, without the TCC working, the engine is wasting 200 RPM in slippage ALL THE TIME ??? I can see it wasting 200 RPM in slippage momentarily when you punch the throttle or when you start to climb a hill, but you are saying that without the TCC, there's a constant 200 RPM of slippage when you're cruising on the flat at a steady 50 MPH ??? In other words, all the numbers on the RPM spreadsheet are based on the TCC being engaged, and if the TCC isn't engaged all the RPM's would be 200 higher. Is this what you're saying?
richard7
11-11-2003, 06:18
Yes. There is slippage when the TCC is not engaged, in all the gears (that's why we don't need a clutch...). There is slightly more slippage when you ask for more power. This slippage is waste energy costing fuel economy and adding a lot of unwanted heat to the transmission oil.
This condition will affect the indicated RPM vs the calculated RPM the chart is giving you. You most probably have a 4.10 rear ratio, which would give about 2250 RPM at 65 MPH, IF the TCC is engaged. Without it, 2500 is logical, dropping fuel economy quite a bit.
TurboDiverArt
12-30-2003, 06:58
Originally posted by Bill H:
When I tap the brakes at 65 MPH or even 50 MPH the RPM does not change. The thing I have trouble understanding is that at 50 MPH, without the TCC working, the engine is wasting 200 RPM in slippage ALL THE TIME ??? I can see it wasting 200 RPM in slippage momentarily when you punch the throttle or when you start to climb a hill, but you are saying that without the TCC, there's a constant 200 RPM of slippage when you're cruising on the flat at a steady 50 MPH ??? In other words, all the numbers on the RPM spreadsheet are based on the TCC being engaged, and if the TCC isn't engaged all the RPM's would be 200 higher. Is this what you're saying? Hey, something I think I can finally help someone out with... Bill, a torque converter is a fluid coupler. This is why we don't need a clutch with an automatic transmission. The inner portion of the converter is connected to the transmission with what sort of looks like a fan (very simplistic explanation). The outer housing of the converter is connected to the engine. As the outer housing rotates faster it's harder for the fluid to flow through the "fan" and the transmission begins to experience torque and spin.
At high torque there is slippage, this is a torque multiplier but not important right now. At cruising there is still a small amount of slippage. As others have noted, it's about 200 RPM's. This means that the engine is spinning about 200 RPM's faster than the transmission input shaft. This naturally is expending more fuel than is needed, it also heats up the transmission fluid as it beats around in the converter, especially when towing. Now enter the lockup. A lock-up torque converter has a lockup clutch in it. Based on an electrical signal from your computer it will lock up the clutch in the converter when in Drive or OD under light loads when above a programmed RPM. What this does is create a one to one ratio between the engine and the transmission. This means the engine is going to spin about 200 RPM's slower at the same speed when not locked up. Now the engine output RPM = Transmission input RPM ad saves fuel and stays cooler.
The TCC will disengage when the transmission shifts back into 2nd gear, when you increase the torque load (hit the throttle), go below a certain speed or when you tap the brake. So, if your TCC is working properly, when cruising at say 60 MPH on the highway without anything in tow and not accelerating in OD, you should be able to lightly tap the break, enough to light up the tail lights but not hard enough to brake and you'll experience an increase in RPM's of about 200.
If you are cruising on the highway and you tap the brake and there is no increase in RPM then try this (I think you have already). Try increasing speed quickly but not enough to switch out of OD and watch closely the RPM's. If you accelerate in OD quickly and you don't see an increase in RPM's then maybe your TCC isn't disengaging the way it should be. Without the TCC engaged you should see an increase in RPM's that are not in proportion to the MPH you are accelerating. If it
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