08 with horrible mileage

[crazydave01]

Well after much research I decided to purchase a duramax. 08 GMC CCSB with 65000 miles on the clock.

My previous vehicle was an 06 Silverado I purchased brand new with 4:10 gears and a 5.3L ext cab. I got 15.5 avg economy with it. Not bad. Then I bought a v nose 12' enclosed trailer that weighs 2880 lbs. I haul my motocross bikes in it. The 06 mielage dropped to 9 - 10 mpg hauling it and I felt I was pushing the truck too hard going 70 - 75 on the freeway.

Enter the 08. It has a front leveling kit, 305/50/20 tires (32" tall). Other than that bone stock. I bought it in Missouri and drove home to Ohio. From my research I was expecting 17 MPG on the drive home. Best I got was 14.8 from a tank and averaged 14.2. That was with the cruise set at 75. Ok, head wind, bad gas, it will come around. NOT. I am averaging 11 -13 around town and highway mix.

I took the time to put a tank through it driving as easy as I could, got and average tank of 12 MPG. It was not worth going so slow with no gains.

So today I hauled the trailer same as I always did with the gas truck. The same trip and same trailer with the same equipment in it. The gas truck got 10 mpg and the new to me diesel got 8.2 !!!!!!!!!

I want to sell this truck ! Not only is it getting WORSE mileage than my 06 gas truck but diesel is .30 a gal MORE !

Can ANYBODY explain to me where all these people seem to get these ridiculous mileage claims ? I simply no longer believe them.

All of my mileage calculations are achieved by using my Garmin GPS for mileage and the amount of fuel pumped in as told by the fuel station pump.

[Kennedy]

Having the nose in the air with big wide tires does not help.

Fuel quality can have an impact.

Does the speedo match the GPS speed?

[Mark Rinker]

Sounds like the tires are costing you 2mpg loaded, 4mpg empty. The more start and stop driving you do, the worse it gets, right?

[crazydave01]

The GPS reads about 3 -4 mph more at 70mph than the speedo and the miles are also more than the truck shows at each fill up.

I understand the tires cause a little loss and the front is only a couple of inches higher than stock so I cant imagine this causing my 8.2 mpg towing a little trailer. Especially when my smaller gas truck got 2 mpg better under the exact same conditions. The only reason for purchasing the diesel was for fuel mileage especially when towing. Seems I have shot myself in the foot.

I do have an ECM coming tomorrow from you John. What I am worried about is installing the new tune and getting better mileage but not repairing any real problem first. I mean by this is if I get 17 mpg empty with the new tune and I am happy and leave well enough alone what if there is an underlying problem and I should be getting 19 - 21 ?

[crazydave01]

Sounds like the tires are costing you 2mpg loaded, 4mpg empty. The more start and stop driving you do, the worse it gets, right?

I cant imagine these tires costing me 4mpg. I could see if they were 35 or larger but they are only 1.5 larger in diameter than stock. I'd like to see data showing the tires size and the power loss created. If they cost me that much I can change the gears to 4:10 and get the final drive back to where it was.

[Kennedy]

The GPS reads about 3 -4 mph more at 70mph than the speedo and the miles are also more than the truck shows at each fill up.

I understand the tires cause a little loss and the front is only a couple of inches higher than stock so I cant imagine this causing my 8.2 mpg towing a little trailer. Especially when my smaller gas truck got 2 mpg better under the exact same conditions. The only reason for purchasing the diesel was for fuel mileage especially when towing. Seems I have shot myself in the foot.

I do have an ECM coming tomorrow from you John. What I am worried about is installing the new tune and getting better mileage but not repairing any real problem first. I mean by this is if I get 17 mpg empty with the new tune and I am happy and leave well enough alone what if there is an underlying problem and I should be getting 19 - 21 ?

That is a start. How wide are these tires? They sound like steamrollers. This does not help. Rolling resistance can add up quickly. How much pressure are you running in them?

The ECM should put the speedo in line with the GPS.


With the ECM/DPF test pipe you always have the ability to return to stock. My line of thought is tio start with the ECM and exhaust mods as they have been shown to give nice improvement. If they don't make a big difference, then check the catalyst for plugging etc. Keeping the mods simple allow you to easily revert to stock.

Be sure to protect the vacant electrical plugs of the DPF temp sensors while you are "testing" the DPF.

[crazydave01]

Well i have installed the Kennedy ecm and a magnaflow 4" turbo back off road exhaust system. I made the same trip again, same trailer , load and time of day. Same weather conditions luckly also. Net gain ..... 3.1 mpg. Went from 8.2 to 11.3.

I know what your thinking, nice gains ! So am I. BUT and I say this loudly BUT I am still extremely disappointed in that figure. That is only 1 mpg better than my gas truck was and I beat on it to go 70 -75 pulling the same trailer. Not to mention gas is 30 cents a gallon cheaper. I consistently see reports of people pulling 12k# travel trailers getting 12 mpg. Thats 3 times the weight of my trailer.

So.... I dive into this further. The air intake valve on my LMM motor had values that were a little high according to the dealer ( I had a check engine light on when I bought the truck ). The mounting ear was broken so they would not warranty it. I received the new the other day and decided to put it on. Here is what I found.....

http://picasaweb.google.com/canalfultonent/08DuramaxIntake#5557315016704467170http://picasaweb.google.com/canalfultonent/08DuramaxIntake#55573150167044671701834

It may be hard to see but there is a crack leading from the egr port mounting flange up through the intake tube. NICE. Here is a close up.

http://picasaweb.google.com/canalfultonent/08DuramaxIntake#55573150748387348981835

Needless to say especially under a load the pressurized air that has already been metered is escaping. Now I am no tuning guru but I know enough to say that is effecting my towing mileage. I dont know if it will gain very much but I am hoping after replacing these parts I can get up to 14 hauling my trailer.

In this picture you can see all the soot the was forced out showing the evidence that caught my eye. It is hard to see also but the left egr port has a threaded hole for a stud to hold the intake tube down and it is broken off also.

http://picasaweb.google.com/canalfultonent/08DuramaxIntake#55573154310981645301836

Unfortunately the dealer wont cover these parts either as there is this broken mounting tab and one on the intake tube also. The truck was in an accident before I bought it which I was aware of, A new front clip and a new frame repaired it as good as new. The warranty is still in place but the dealer wont cover these parts because it was obviously damaged in the accident. Good news is the seller is taking care of the parts for me !

The other issue is the pcv system. I am going to install 2 oil separators ( one for each valve cover ) to eliminate the oil going into the system causing this build up at 65000 miles

http://picasaweb.google.com/canalfultonent/08DuramaxIntake#55573155970738045461837

1838

http://picasaweb.google.com/canalfultonent/08DuramaxIntake#5557315560524494834

http://picasaweb.google.com/canalfultonent/08DuramaxIntake#5557315478952496146

This has got to affect the performance and mileage of the truck also ! Yuck !

[Mark Rinker]

Also - any towing mileage measurements should be taken at whatever ground speed 1800rpm will buy you.

If you are towing >70mph and expect mileage above single digits...well...dream on.

[crazydave01]

Already got 11.3 towing at 75 mph ...... Hand calculated using gps.

[Mark Rinker]

Already got 11.3 towing at 75 mph ...... Hand calculated using gps.

Great news. If you were towing at 1800rpm, you'd likely be getting 13-14mpg.

[crazydave01]

And since I have 32" as opposed to the factory 30.5" tires that is exactly what my rpm is. This is why I am disappointed. As you stated I should be 13 - 14 mpg.

[Mark Rinker]

And since I have 32" as opposed to the factory 30.5" tires that is exactly what my rpm is. This is why I am disappointed. As you stated I should be 13 - 14 mpg.

The reason you will never see optimal mileage at 70-75mph towing speeds has more to do with wind resistance than tire size.

By increasing the tire diameter from stock, you shouldn't be getting optimal mileage, and you aren't - but that is a matter of choice, made when you varied the vehicle from stock trim. An easy test would be to put the stock rims and rubber back on for awhile and see the difference...and slow down.

Everything about the Duramax EMC and TCM is based on stock rubber and 3.73 gears. As soon as you alter that, you are losing efficiency. Telling the ECM what your new tire diameter is fixes speedo error only. It does not optimize gear ratios, shift points, rolling resistance, fuel maps, injection timing, etc.

Also - the LMM also uses about 1 gallon per tankful on regeneration of the DPF, regardless...hopefully you have removed that variable. If so, that may be the easiest mileage increase you will find.

[crazydave01]

Also - the LMM also uses about 1 gallon per tankful on regeneration of the DPF, regardless...hopefully you have removed that variable. If so, that may be the easiest mileage increase you will find.

I realize I will not get OPTIMAL mileage. But I will get better than 11.3 mpg. Obviously you are more bent on proving me wrong than giving your 'expert' advice. I clearly stated that I had installed a kennedy ecm and 4" magnaflow off road exhaust. I had already posted I was getting 11.3 mpg at 75 mph when you told me to "dream on" if I thought I would get better than single digit mpg towing above 70 mph.

If you seriously think that the broken intake pipe and egr ports that are letting metered air escape doesnt affect the mileage then you are certainly no person to be helping here.

I am sorry if my high expectations cause me to strive for the best out of myself and my truck. Maybe I wont get 14 mpg hauling my stuff at 75 but if I'd listened to you 8.2 would have been good enough. Now 11.3 should be. Great if it is enough for you but that just isn't for me. I'll keep you posted as my mileage increases. I haven't cleaned the MAF yet, I haven't bumped up my tire pressure yet, I haven't changed to all synthetic oils yet. I haven't experimented with fuel additives yet. And most of all I haven't fixed my broken intake parts yet. These are the type of suggestions I was looking for and found by research here , research on other sites and from John Kennedy. Certainly wasn't expecting ... " dream on " !

I don't mean to sound harsh but If I had listened to everyone who told me I couldn't do something I'd probably still be flipping burgers at McDonalds. I probably wouldn't have built a 5.6L DOHC Navigator motor and stuffed it in my 99 Mustang, after fabricating a new intake and front cover so I could mount the factory 03 cobra supercharger on it. I sure I wouldn't own my own electrical contracting company either, giving me the luxury of doing all these things......

Besides If I understood John Kennedy correctly he thinks I will do a little better than 11.3 also.

[Mark Rinker]

You'll need some thicker skin to thrive around here. :D I am not an expert at anything, but have about 600K realworld on Duramaxes, mostly towing, so I have an informed opinion.

Welcome to TDP.

[Kennedy]

And since I have 32" as opposed to the factory 30.5" tires that is exactly what my rpm is. This is why I am disappointed. As you stated I should be 13 - 14 mpg.

75mph is 75mph is 75mph


It's not the RPM that gets you it's the wind. For every 5mph over 60mph you will typically lose 1-2mpg.

OK with that out of the way, it looks like you are seeing the beginning of some decent MPG gains. Fixing the intake issues will only help. They will probably not make huge leaps and bounds believe it or not, but you just won't know until fixed.



P.S. The speedo should be correct on the tuned ECM, but Id suggest you verify versus GPS just to be on the safe side.

[crazydave01]

Speedo is fixed. At 75 it is dead on. The weird part is below that it shows about 1 mph slow. As close as my other truck was from the factory !

The truck drives 100 % better now also. The shift points are great and it coasts to a stop better instead of the small tranny braking it had when not loaded. The power is much nicer. The tune is great. I will keep you posted on improvements or lack there of either way.

Thanks for all your help John

[Kennedy]

You've stumbled onto a strange phenomenon. I really get a kick out of when a guy tells me his speedo is "3mph off". It's not. It's a percentage across the board. An error 3mph at 60 is 5% so it's 1.5mph off at 30 etc. It's pretty simple for the most part, but there is a catch that I'll get to later. The Output speed sensor has 40 teeth teeth and it spins a specific number of pulses per mile. Gear ratio and tire size factor in here.


What I find (assuming that the GPS never lies) is that the speedo head is a bit quirky. It seems that a guy can be dead nuts on at 50mph and 70mph thet at 60mph it is off. I'm not sure if this is a calibration issue or what? Maybe there is a deliberate "fudge" going on here. It seems that in the 60 mph area it often reads 1-2mph fast. Could this be deliberate?


Glad your truck is turning around and improving. I think it will get better yet as things get sorted out. I can tow my 2002 in pulling trim (8,000#) on my car trailer (Mid 33ks) and run in the 13-14mpg range, BUT not at 75 mph. Going west to numerous DP events we were in the 11mpg range. Much of this was back before I was doing my own tuning though...

[crazydave01]

I thought it was weird to. I knew it was a glitch of some sort because i am familiar with the ratios you explained. Like I said though it is as accurate as my stock 06 truck was. I am very pleased ! I think I will do a lil bit better. We shall see how it goes !

[crazydave01]

Also - any towing mileage measurements should be taken at whatever ground speed 1800rpm will buy you.

If you are towing >70mph and expect mileage above single digits...well...dream on.

Just took a trip to Alabama. Left ohio the morning of a HUGE snowstorm, averaged 30 mph in 4WD hauling just over 5.5K lbs. The first tank avg was 9.3 mpg. Cleared up the rest of the drive and got 12.8 mpg with the next tank of fuel. The last tank we got 12.4 mpg but had run into rush hour traffic n Atlant on that one. These two tanks were both with the cruise set at 75 mph as much as possible.

Ran a tank out down there with no trailer avg of 14.2 mpg. The drive back had bad rainstorms and horrible head and side winds. Three tanks avg 11.8, 11.4 and 11.5 mpg.

All of this hauling over 5500 lbs at 75mph as much as possible. Before we left I changed all fluids to synthetic, 60 psi in all tires ( The previous owner bought cheap tires and that is all they are rated for) and cleaned the MAF.

Looks like I am as close to 13 mpg hauling my trailer at 75 mph as I am going to get. But it is a FAR from where I started and a good bit above those " single digits " I was DREAMING about. Glad I don't settle.

[Mark Rinker]

Winners never settle.

My trailers weighs ~3500#, empty. Loaded averages (trailer + cargo) are around 12-15K#. Lots of time 12' wide and 14' tall, so wind drag factors in as well. As for the "single-digits at 70mph" reference, I was assuming you were towing something heavier.

Congrats.

[Kennedy]

I'd put some REAL tires on also. Something like Michelin LTX MS in a 265 range E.

Glad the ECM tuning is helping