6.5 vs. 6.2 mileage [Archive] - TheDieselPage.com Forums

jbell

07-01-2004, 12:44

I've searched post after post, and it seems that in general, 6.2's get great mileage, and 6.5's get only fair mileage.

Why?

It seems that the general concensus is that 6.2's (turbo or NA) get 18 - 26 depending on the vehicle, driver, final drive ratio, etc... and 11 - 16 on 6.5's.

I personally get 20mpg average in a 3/4ton suburban with 4.10 gears -- loaded with vacation stuff, doing a combination of running 80mph on interestate, city, and mountain driving. (The way I drive IS NOT the way to get good mileage BTW.)

Several of the 6.2 upgrades are 6.5 parts, which makes this seem even stranger.
Any thoughts?

Shuck

07-01-2004, 12:57

More HP/torque = more fuel consumption. That energy has to come from somewhere. There are a few things that can be done to make an engine more efficient, but in general, to get more power out of an engine, you have to add energy and that is usually done in the form of chemical energy (fuel).

If I remember right, wasn't the 6.2 rated at something like 89HP? I used to have a 91 1/2 ton pickup with a 6.2 and it felt like about that. I could hold 35mph towing my boat up a hill. My 6.5TD suburban, on the other hand, has no problems holding any speed up a hill. I think it is rated at 215HP (it's a 99).

AndyL

07-01-2004, 13:35

The J code 6.2 was rated at 155HP. The C was maybe 10 or 15 less.

matt-max

07-01-2004, 17:59

highway driving at steady, reasonably legal speed with a/c on nets me almost 20mpg in the yu-hoe which has 3.73's and weighs 5800#. last time i checked it got 19.5+ mpg at 67 mph

(3500hd doesn't break 10mpg but has 5.13's and weighs 9200# empty, which it never is)

matt

JohnC

07-02-2004, 06:33

One big factor is the size of the pre-combustion chambers. The 6.5 chambers are designed for more power and turbocharging. there is a tradeoff on efficiency.

Shuck

07-02-2004, 07:18

Well, my old 6.2 had 178k miles on it with original EVERYTHING, so it was probably making about 90HP =)

EscaladeDiesel

07-02-2004, 09:19

I don't get anywhere near that on my diesels. Granted I drive 70 to 80 mph, but I am lucky if I got 16 on my 1998 and about the same on my 1983's. Seems I remember reading somewhere that fuel efficiency plateaus at about 65mph especially with the 4:10 in my '98 not sure what my '83 3/4 4x4 has, but I would imagine about the same.

cudaaa

07-02-2004, 17:55

I constantly got 15-16 mpg. When I put the heath 2.0 chip and turbomaster on it jumped to 18-19
and a lot more power! cudaaa

Dart

07-03-2004, 18:01

With a 97 C2500, automatic with 3.73 gears extended cab long bed.

Up to 21.5 mpg keeping it below 65 in mountains.

Usually, about 19 combination freeway/city.

Only modifications, Bd exhaust and downpipe, air filter.

Been getting about 20 +/- using Stanadyne fuel treatment lately.

Considering going to taller tires, to change rear end ratio a bit, to slow engine down a little.

jbell

07-03-2004, 18:53

http://forum.thedieselpage.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=2;t=002918

This guy gets better than me by a long shot, but I'm happy with 20 I guess. I have a 1985 3/4 ton suburban, "j" code 155hp stock 6.2, 4.10 gears, 265-75r16's, and I swapped the th400 for a (gasp) 700r4 from a rusted out 89 burb. (I had to get the rpm's down to a reasonable level.) I'm still nowhere near optimum rpm's at speed, If I had 3.42's I guess I would be getting mid 20's, but I'm happy with the pulling and passing power of having 4.10's. And, as I said, the way I drive is not the way to get good mileage. If I'm easy on it, I get much better. (If I drove 67mph steady.... Nope, not me.... I'm part of the North America Left Lane Gang.) With the th400 and 4.10 gears - 16mpg was the best I ever saw, but then I was running WAY over 3K just to try to reach the speed limit. Now I'm around 2300@75mph.

The only two things that make sense, are the upgraded cooling system in the 6.5 (more fuel energy goes out through the radiator instead of the tires maybe?) and the post about the IC precups is interesting as well.

As I said in my original post, I just don't get why 6.5's don't get 20 or better, unloaded, running 80 down the road. Any (I'm assuming) 6.2 with an OD tranny and lockup converter can.

Is the tranny the culprit? I know conventional hot rod wisdom says that a th400 burns 13hp more than a th350. Most (maybe all) 6.5TD's have a 4l80e which is a TH400 with OD. Is the reason I get 20, with my foot in it, and the newfound speed that I have, due to the 700r4? and more power making it to the ground? (yes, I used speed and a 6.2 in the same sentence.)

MEdlin

07-03-2004, 19:47

I have a 1994 3/4 ton Suburban with the 6.5TD and 4.10 rear end, running Michelin LTX M/S LT245/75 R16 tires. Im scared to run at 80 mph for fear of blowing the thing up !!! As I understand it, the optimal RPM for a diesel is around 1900 when crusing. I don't have any problem running at 70-72 mph, but my RPM is up around 2800. I generally get between 15-16 MPG overall. My best was only 17 on the highway. Does it make sense to make Chip changes, etc on a truck with 177000 miles in search of better milage? smile.gif

jbell

07-04-2004, 09:37

Medlin:
RPM = MPH * Gear Ratio * 336 / Tire Diameter
Your 245 tires are 30.5" tall.

I can't figure out how you get 2800 @70mph?
A 4l80 is a .75 OD, 7004r is a .69

70*4.1*.75*336/30.5 = 2371.

The only thing I can figure is that you don't have a lockup torque converter and you have about 400rpm slip. (typical slip is 200 - 400 rpm)

Is that the big difference? Do the 6.5TD's not have lockup? If so, that would account for the big mileage difference.

MEdlin

07-04-2004, 10:20

jbell - I do have the lockup torque converter, but I did replace the alternator about a year ago. Maybe it has the wrong sized pulley. Do you know the correct size for the alternator pulley?

moondoggie

07-05-2004, 03:38

Good Day!

Original question: I've wondered the same thing. My 82 pickup (1/2 ton, 3.08 gears, 4 spd manual w/ OD) ALWAYS returned 26 - 28 mpg in the summer when I 1st bought it. (Please keep in mind, I'm one of 3 people in MN that don't speed.) November 02 I pulled it out of 3 years of storage (needed a vehicle fast [not a fast vehicle]) & ran it through about March & it still returned ≥ 24 mpg.

My 95 pickup mpg can be seen in my post in What's your fuel mileage with a load? (http://forum.thedieselpage.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=1&t=004439#000014) (Click in colored text.) My effective overall ratio with the GV OD & 4% taller tires is 3.08

G. Gearloose

07-05-2004, 06:30

I don't drive like an old lady, I'm so slow the old ladies give me the finger!
And my '96 K1500 xcab 4l80 never gets better than 18 mpg

I've had 3 other 6.2, K5, K20's, 3.08's, 4.10's, 235's, 35" mudders, 700R4 or M465, Banks and NA, turned up pump, etc.

They all got better milage than the 96, by 3-5 mpg, and weighed within 500# and had worse aerodynamics.

I suspect a .2 CR change in pre-cups can't make that much difference if people are running 18:1 pistons without a big milage hit..

Does the 4l80 have 6-gear planetaries in the OD as well as the 1 and 2 gears? The 700R4 has more typical 3-gear planetaries. Could that be where the efficiency is going?

We've pretty much eliminated gear ratio, My best milage truck was M465, 4.10's and 235 tires.

Was the biggest milage hit in the '94 or '96 model years? (ODB1 & 2)

Was higher rpm timing cut back to lower NOx?

Does the 6.5 with MFI suk with milage too? If not, did any 6.5MFI's get the 4L80 instead of 4L60? When did the 4L80 become the Std automatic? Perhaps we can narrow down the culprit.

jbell

07-05-2004, 07:16

I think that the 4l80 may be what is taking the extra power / fuel. I can tell you that my 3/4ton burb was slower than a school bus. (Remember 4.1 gears, no OD -- it should have had great 0-60 times.) I put in the 700r4 and now mileage / acceleration is great. If I could actually keep it under 70 for a tank (all 41 gallons) I'm sure I could post numbers much better than 20.

I thought all 4l80's were lockup, thanks for the confirmation, I thought maybe gm did something stupid, and eliminated it on diesels for some reason on MEdlin's post.

Anyone have a fuel curve for a 6.2 or a 6.5 like what cummins provides? (It gives fuel in pounds/hr per horsepower throughout the rpm range.) That would tell us if the 6.5 is just generally less efficient, or if the drivetrains behind the 6.5 are less efficient.

G. Gearloose

07-05-2004, 07:24

just remember jbell that the 700R4 has a much steeper first gear than the other automatics, so any seat of the pants acceleration comparison will be skewed to the 700R4.
What was the previous transmission?

jbell

07-05-2004, 11:14

Previous tranny was th400, no lockup.

Yes, 700r4 has steeper (3.06) 1st gear, but the needle climbs fastest in 2nd, and 3rd, not 1st.

The biggest problem the 700r4 has had is the big jump to 2nd. Most gas engines drop rpm below where they really make power on that shift -- and that causes acceleration problems below 50. The 6.2 has the low end torque to power through the big 1 - 2 shift.

I'm no fan of the 700r4 (went through 6 of them in the early 80's in an s-10) And in this one, I've given up hard launches, as I can feel a little 2nd gear slip when it shifts. But 70 at the end of an onramp is no problem.

Seat of the pants -- this land barge is as fast as my v6 tacoma on 32's (and gets the same mpg.) The only thing I don't have is uphill passing power. (I'm working on that......) Maybe that has something to do with the tacoma's 330,000 miles vs. the 157,000 on the 6.2 -- don't know.

Andy Chesek

07-05-2004, 14:27

I have a '99 K2500 pickup with 3.73's and the NV4500. Just came home today from hunting camp (Coudersport, PA to Dover, PA = ~ 220 miles), and averaged 21.5 mpg while cruising between 65 - 70 mph at most times. Last time I made the trip I got about 20.2 mpg.

Right before this past trip I changed my transmission oil to Mobil 1 Synthetic, and added a little Lucas Synthetic Oil Treatment to both the tranny and engine. I also added Diesel Service's Power Kleen (gray bottle) to the fuel. Last difference between the trips is that I added a TurboMaster.

My Pinnacle 4" exhaust kit should arrive in the next two days, and I just received a Dodge Ram Intercooler which I scored off eBay a month ago. Can't wait to see what happens once I get those installed... I love my truck, and this site rocks.

Forgot to mention... running 245/75R16 BFG All-Terrains and only had a few mountain bikes, a cooler, and some clothes for a long weekend in the truck.

Andy

[ 07-05-2004, 05:34 PM: Message edited by: 93_Burrito ]

jbell

07-06-2004, 05:32

good mileage and a manual nv4500 -- is that another vote against the 4l80 and good mileage?

Shuck

07-06-2004, 10:14

A manual will almost always beat an automatic from a fuel economy standpoint. They waste less energy in heat and they usually have more gears which means the engine can run in it's best ranges more (torque converters help this, but then counter balance it by generating a LOT of heat).

The VW TDIs have an 8mpg difference between auto and manual transmission - I imagine our bigger engines help minimize that difference, but I bet the guys with manuals are beating the guys with autos in the mpg competition. Of course, those of us with suburbans are hosed - GM never offered a suburban with a manual.

jcomp

07-06-2004, 17:06

My truck with a 200K mile N/A 6.2 got 16 MPG coming from Denver to here (about 200 miles), lots of mountains (at least 3 different 9000+ ft passes) and 65 MPH most of the way in drive.

The same engine with the banks kit got 19 MPG on the same stretch, running in drive.

My current engine, new (rebuilt) 6.2 short block, 6.5 heads, 4911, 6.5 injectors, 6.5 dual t-stats, banks turbo, gets 17MPG on that stretch in overdrive, 19MPG in drive.

My truck perfoms better and gets better economy at 2400 RPM (drive) than at 1700 rpm (overdrive). My EGTs also stay a few hundred degrees lower.

I think that the gearing being too high (numerically low) is probably a cause for some trucks getting poor mileage. The engine needs to run where it is most efficient.

jbell

07-07-2004, 07:21

yea, optimum mileage is attained driving @torque peak, not below. 1700 is definately WAY too low, should be 1900-2100 for best economy. I'm too high on rpms, but that is much better than being too low.

Also the rolling resistance of 37's..........

moondoggie

07-08-2004, 06:19

Good Day!

Shuck said,

jbell

07-08-2004, 07:15

moondoggie,
If mpg is your thing -- you got any of the fuel curves for a 6.2? or 6.5? (like what cummins puts out for their cpl's?)

For example: a B3.9 120hp CPL0986, is flat on fuel consumption from 1600 - 2100 @.37lbs/hr/hp.
At 2100, fuel consumption starts to rise up to .40@2450rpm. So on that motor, you DON'T want to go above 2150 for best mileage, and the best combination of HP/Torque is about 1900 @ 100hp/300ft.lbs (more or less, I'm reading a graph.)

If we had a fuel curve / power curve, we should be able to predict mpg / performance reasonably well.

moondoggie

07-08-2004, 08:50

Good Day!

Nope. In fact, no one that's ever seen me work would use the word "mechanic" & my name in the same sentence. I'm a shade-tree type guy. I have very limited mechanical skills - the only reason I work on these things at all is it's the only way they're going to keep running. Repair shops are WAY out of our budget at this time.

I have additionally noticed with all these trucks, mpg seems to drop off rapidly over 65 mph. We have 70 mph speed limits in a couple places in MN. I run at the posted, so I do 70 most of the time on these roads. I'd almost rather they were 75 or 80, I'd just drop back to 65.

Blessings!

Brian Johnson, #5044

jbell

07-08-2004, 12:34

moondoggie:
No worries, I grew up on a farm, but turned geek. So I'm definately no pro. Shade tree guys are fine with me.

Most diesels have a "smile" fuel curve, with the lowest consumption at max torque, and higher consumption/hp @ lower and higher rpm (like the cummins cpl0598 -- typical breadtruck motor.)

My assumption at this point on the difference between 6.5 and 6.2 is drivetrain friction losses. Drivetrain losses increase with gear pressure, that's why an overdrive, and 4.1 gears are more efficient than a direct trans and higher gears with the same final ratio. So my assumption is that a 700r4 with smaller components and taller OD has lower rotational/friction losses, and due to the 4.1 rear gears has lower 'gear pressure' friction related losses in the transmission. The rear end has lower losses as well, due to the 4.1, vs something higher (numerically lower.) due to lower hypoid gear pressures / higher rpms on the pinion.

I'll let you know how this tank goes, but I've been driving alot of 2lane @#$@#$ 65mph roads, so I'm on track for probably 22-23, I know at half tank I've gotten alot more miles than typical. (that's pie in the sky numbers for a 6.5 / 4l80.) I've 'heard' that you lose about 1mpg per 5 mph above 55, sounds about right.

I know this: I wouldn't dare tow 10,000 with a 700r4, I would with a 4l80 or an allison. So maybe that's a good tradeoff vs. mpg.

norm

07-08-2004, 17:15

I guess I'm destined for 12mpg... I do have a crew with a barn-size Reading body out back. Definitely glad I don't have a rat motor anymore (my '85 K30 454 was fun but 8mpg not fun).
What kills me is my buddy with a lightning fast Cummins with only exhaust and injector mods (puts you back in the seat like a muscle car) and he can cruise at 20mpg and average 15!!

jbell

07-08-2004, 18:20

Norm:
12mpg? I keep hearing that over and over, and there's no reason for that. Even with my ramblings about friction, etc.. Your buddies cummins should get about 15% better mpg due to DI vs IDI, but 12 to 20 is a huge difference.

I'm a 3/4 ton burb, so I'm heavy, maybe not as heavy as with your utility box when empty, but loaded down with 41 gallons of fuel, family, and vacation stuff -- reasonably close, if not higher (You haven't seen the way my wife packs....)

You also have the same rear axle (14bolt, 4.1 gears) and same tires. (265-75r16's) You've got a 3" exhaust, I've got dual 2.5's so you have the better exhaust. The only difference is the 4l80 and the 6.5.

The guy I bought the burb from got 13-14 all the time towing a 26' airstream, 15-16 empty with the th400...

You shouldn't be getting 12.....

norm

07-09-2004, 17:26

I figure I am around 8.5K without "cargo" - and a much worse drag coefficient than a suburban. (tall, covered util. body) My truck is my service truck and I have everything from tools to refrigerant/oxy-acet./comp.nitrogen/ etc. bottles to duct tape and band aids... Now I know that's heavy for a single unit, but it is not like a 7+K pound truck pulling a 8k trailer! So go figure. I do have a friend who used to have a 6.2 (stock) and said he got 20mpg "no matter what".

jbell

07-10-2004, 10:44

I would agree with the 20mpg, no matter what. Finished off this last tank, only 21 -- I thought the slower roads would make more of a difference, but it didn't.

Here's a good one though, check out current gas suburbans, with the 5.3 and 6.0 motors. I know there can't be THAT much efficiency difference between those two motors. The total weight difference between the 2500 and 1500 is very minimal. However, gm posts NO fuel mileage numbers for the 2500 with the 4l80, but posts 15/20 for the 5.3/4l60 trans sub.

Hmmm. Think gm is embarrassed about those numbers? (Have a friend with one -- mileage is embarassing -- 12mpg average, 7 towing.) No where near 15/20.

I did some math to convert mileage numbers into HP requirements. In order to do that, I had to 'guess' on the fuel/hp consumption on the gm diesels. I also have to 'guess' that the 6.5 and 6.2 can make similair numbers. (folks with 6.2 block, 6.5 heads, 6.5 injectors, etc... making 20mpg with a 700r4 trans.)

With all of those guesses, this doesn't mean a thing, but here goes:
cummins 4bt motors make about .38lbs/hr/hp @1900rpm, DI. A IDI engine is about 15% less efficient, making a 'guess' @ .437lbs/hp/hr. for our gm diesels.
Diesel is 7.6lbs/gallon.

If I arbitrarily say a 6.5 sub makes 15 @80mph and the equivilant 6.2 sub makes 20 @ 80mph, both geared the same. the hp requirements are:

80miles / 20mpg = 4gal * 7.6 = 30.4lbs / .437 = 69.5hp required.

80miles / 15mpg = 5.33gal * 7.6 = 40.53lbs / .437 = 92.75hp required.

The hotrod sites claim that a th400 takes 13hp more than a th350. (The trans. that the 700r4 and 4l80e are based on) Nothing to validate that, also nothing saying what rpm, what load, etc... so take that with a grain of salt. The above difference is 30.25hp -- hard to believe that a 4l80 can waste that much hp..... But, if you believe that a 6.5 and a 6.2 can make the same hp on the same fuel -- something has to account for the fuel difference.

Anyone anywhere have a 6.5 running something besides a 4l80 auto trans?

jcomp

07-10-2004, 12:04

One other thing that may be of note is that my injection pump is turned WAY down. It was freshly rebuilt when I bought it for this engine, and I had to turn it down 1/2 turn to keep my EGTs in line.

moondoggie- my 19 MPG measurement was based on a 230 mile trip. I've only got 1200 miles on this engine, so I haven't had much opportunity to track the mileage over a long term. When I get more data I'll post it.

Right now though, my truck is on the injured reserve list. Its brakes need my immediate attention, and I've decided to swap in my 1-ton diffs while I'm at it. I'll probably also throw in some 4.88 gears for good measure. :D