4:10 Gearing and Fuel economy

[akskycowboy]

Hello all,
I purchased a 1996 GMC Suburban 2500 4X4 with 76k original miles, 6.5L and 4:10 gears. The guy I bought it from told me that when he set the cruise on 70mph or just below, he was able to get 20mpg. Well, I bought it and drove it home, 6 hour trip, and two days of light driving around town, filled up and crunched the numbers and only got 14 and change mpg. He had told me also that the worst he ever got was 15mpg. So, my question is the gearing, if I was to run taller tires, say 35's, would I get better fuel economy on hwy driving? Any help on improving fuel economy will help. Mind you, I wanted a 7.3L Excursion but saw this vehicle as a good compromise for savings on initial cost and over the life of the vehicle.

Thanks

[greatwhite]

Hello all,
I purchased a 1996 GMC Suburban 2500 4X4 with 76k original miles, 6.5L and 4:10 gears. The guy I bought it from told me that when he set the cruise on 70mph or just below, he was able to get 20mpg. Well, I bought it and drove it home, 6 hour trip, and two days of light driving around town, filled up and crunched the numbers and only got 14 and change mpg. He had told me also that the worst he ever got was 15mpg. So, my question is the gearing, if I was to run taller tires, say 35's, would I get better fuel economy on hwy driving? Any help on improving fuel economy will help. Mind you, I wanted a 7.3L Excursion but saw this vehicle as a good compromise for savings on initial cost and over the life of the vehicle.

Thanks


Either his math is wrong or he flat out lied to you.

14 MPG is normal for that configuration.

3:73's and 20 mpg would at least be more believable. 3:42's more so, but that would make your big heavy rig "a dog" for sure.

My 98 K2500 TD is the 8600 GVWR config. 4:10 and auto. It's an ECLB so it's comparable to your Sub.

It's got a new optimizer 6500 and everything else under the hood is (literally everything except the rebuilt IP) less than 3 years old. Turbo master, custom flash, K&n, synthetic fluids in everything right down to the differentials.

I'm running one size larger than stock BFG AT tires.

13-14 MPG is a normal MPG around our small town driving.

70 mph of a K2500 is going to drag the mpg down for sure.

If I keep it around 55 mph (90 kph for us) I can see between 16 and 19 mpg depending of winds and what not.

20 is a number I have never seen in my truck.

Larger tires will reduce your overall final gearing ratio, but you are going to pay a penalty in weight and rolling resistance. Bigger tires put weight in the worst possible position to work against the engine power: at the outside of the wheel. Not to mention, larger tires means a wider contact patch at the road surface which also increases road drag.

Wish I had better news for you, but there it is.

Biggest effect you can have on MPG is to simply drive slower on the highway and keep acceleration nice and light. as in: "Egg under the throttle pedal" light.

Other than that it's the standard things like making sure the tires are at pressure, fresh air filter, free flowing exhaust, etc....

Cheers

:)

[DaveNY]

Same Truck, avg 15+ combined. Never over 70 mph. If I slow down to around 60-63 I notice an improvment? of 16.5 almost 17 for my best.
Dave

[phantom309]

.

3:73's and 20 mpg would at least be more believable. 3:42's more so, but that would make your big heavy rig "a dog" for sure.


I have 3:42's in the k2500 8600lb ext cab 8ft box truck,.It has 285-7516's
It definitely isn't a "dog" ;)

My c3500 4dr dually has 4:10's, 215-85-16 tires, it,s way worse on fuel,.

I think it all has to do with the GMx turbo's and their drive pressure after 2000 rpm,.

I read that swapping to and aftermarket turbo a lower pressure higher volume unit, the first thing folks mention is it's better on fuel,.

From my own personal experiences, i turn up fuel delivery and timing on every diesel i,ve owned, and i always get better performance , but i also always get better fuel mileage. I had plenty of diesels, for personal and business,.
We have a 12.7 detroit that the ecm is set on "stun" with different injectors and a different turbo,. it has the ability to turn the sky brown at any rpm, and pulls pretty hard,.yet if driven "normally" gives excellent fuel mileage,.
Cat diesels are famous for better fuel mileage "cranked up" N14 cummins no different.

My 94 K2500 is one of those trucks that according to experts,. shouldn't run.
It has a 5068 pump on it with optical sensor bump,a GM3 turbo with a "spring" on the wastegate,and a large "cone" air filter..running with the stock 5521 computer chip, and it gives me great fuel mileage if it,s reasonably flat ground, and the wind isn't strong on a 3/4 crosswind etc, I have an aftermarket crossover, a home made larger down pipe,and muffler delete.I cruise at 18-1900 rpm, and it'll do 22+ mpg,. BUT,. thats a big gallon, which works out to about 18+? us gal,. The throttle is very twitchy just off idle tho,.and i can rally hear the turbo 'slurp" when you let off after a hard pull,.
If i run the X brand chip i have , it makes no difference to mpg cruising on the hiway,.( unless there's a lot of steep hills) but around town it drops considerably, because i can't keep my foot out of it,. and boost is addictive.:D

Nick
yrmv
nweoi
pdcc

[AKMark]

My 99 K2500 Burb with 4.10's and 285/75R16's would get 19 if you kept it at 65 or lower. (GPS)

Go 67-72 and it dropped to 17

Go 55 an entire tank and it would go over 20.

Tow 9K with it and it would sit at 13. Idle way too much in -50 temps, and don't tow, and the worst I ever saw was 11.

Granted it had less than 50K miles on Hi-pop marine injectors. Before those were installed, the previous owner complained it was awful.

[Warren96]

I have tried bigger tires on mine, and it made mileage WORSE.It also screws up the anti lock brakes. The best advice i got for my 6.5 was to install the gear drive and use a computer to verify at what speed 1800RPM is.

[AKMark]

Big tires do not screw up the ABS....If the ABS was screwed up it was something else. My speedometer was off but had the GPS and it worked fine.

There are many factors with mpg.....every used truck will be a little different.

[greatwhite]

Big tires do not screw up the ABS....If the ABS was screwed up it was something else. My speedometer was off but had the GPS and it worked fine...

Well, yes and no.

The Kelsey hayes ABS module has a VSSB input in order to know the vehicle speed.

Basically, once it crosses 8 mph (see below, I seem to remember it as 3-5 MPH, maybe that was the WSS test), it runs a system test. if everything comes back A-OK, no DTCs are set and the system carries on as normal.



Once vehicle speed has exceeded 8 MPH and driver has fully
released the brake pedal, BPMV performs a self-check on electro-
hydraulic portion of the system. This includes pump, control valves
and reset switches. If BPMV detects a problem during this self-check
or initial self-test, an indicator light will come on and 4WAL system
will be disabled. A related trouble code will be stored
It also watches for a 15mph threshold for low speed operation. Had a sub in the shop once that had a pulsing brake pedal over 15mph. Turned out the wiring from the VSSB to the ABS module had chaffed and shorted and was the cause of the brake pulse. That was a mother to find.....

Actual lockup actions at the calipers are ultimately controlled by the wheel speed sensors.

Do big tires mess it up?

Not really. It's just off a couple MPH either way for it's low speed activation and system test. By the time you lock up a wheel it's more concerned with the wheel speed sensors anyways...

:)

[phantom309]

I have tried bigger tires on mine, and it made mileage WORSE.It also screws up the anti lock brakes. The best advice i got for my 6.5 was to install the gear drive and use a computer to verify at what speed 1800RPM is.

Lots of people report worse fuel mileage with bigger tires,.

their speedo reads less miles, but they actually go further,..

They fail to realise that their same daily commute didn't get shorter,...:D