Towing and sucking gas like crazy.

[Edward Case]

OK, now we learn something really important for towing.
If you want to move that much weight you really should have at least 4.10 gearing.
I had assumed that a 2500 would already have 4.10 gears.
Is this a "light duty" 2500 with the 6 lug axles?
Pulling 6K with 3.73s, especially up-hill puts you in the area of pedal-to-the-metal in 2nd gear.

over the last twenty years my rear end ratios have risen from something like a 2.73 in a 89 diesel suburban to 3.42 to 3.73 to 4.56 my square body trucks and now 4.10 in my gmt400s. i love pulling with less strain on the motor and trans and dont really kill my mileage unless its running hundreds of miles on a highway unloaded (rare occurance) once you have 2 to 3000 pounds to haul a 4.10 ratio should not hurt mileage much at all.

 

[HotWheelsBurban]

Yeah, I used to have a book that a mechanic at a Chevy dealer gave me, it was about 3/4 inches thick.
It had all the option codes for everything going back into the '60s.
And yes, their was some number/letter for "Full Floating Axle" for the trucks and suburbans.
My present Sub is a K2500 with the 14 bolt 8 lug semi-float, but I've seen 2500s with the 14 bolt FF, it was an option.
Their was a "non catalog" option for Subs that were 9,2000 GVW, and they came stock with the FF, and they had the 3500 front end also.
If you took your time and really pick thru the pages, you could order a really special truck or Sub that no dealer would ever have in stock.

Man that would be a serious tow beast if one could be found! One of the square body Burbs we used to have was a 3/4 ton 454/ turbo 400/ full floater. People we bought it from towed a commercial trailer full of vintage shop manuals, brochures and such all over the country with it. Did the job very well, but when gas got really high, they got rid of it. They'd gotten a diesel pickup by then and it got better mileage so they didn't need the Burb anymore. Dad and I were fixing some things on it, and getting it ready for new exhaust and a fully functional carburetor, and we discovered that it had a lot more rust on it than we'd seen when we bought it....Not sure about the ratio but it was probably 4.10. Truck was good off the line as much as the carb would allow. Hopefully pieces of it went to help out other trucks....

 

[JayMB]

My neighbor who was a firefighter years ago told me that the fire hall he used to work for had a few GMT400 1-ton Suburban K3500's. They were exactly like the 8600 GVWR K2500 Suburbans with 454's, 14FF rears, the same front end (common to 4x4 3/4 and 1 ton GMT400's), the same frame, but heavier rear springs and BW4401 transfercases with PTO setups that ran something special for them. I can't remember if it was a hydraulic pump for a vehicle occupant extraction setup (jaws of life, etc) or if it was air or what.. But it had something on the PTO. I'd love to find one of those. Of course, I imagine an upfitter was involved in that setup.

 

[alpinecrick]

Ok, just a quick calc for us 'Muricans:

Unloaded:
(180km x 1.0 mile/1.6 km) divided by (0.25tank x 34 gallon) = 13.2 MPG

Loaded:
(176km x 1.0 mile/1.6 km) divided by (0.75tank x 34 gallon) = 4.31 MPG!

You said 120 Liters, that's about 32 gallons. I used 34 gallon tank, which is what literature says, but relatively speaking the numbers are off the charts. That sounds REALLY bad, LOL.

OP mentioned '91, that's the first year of the 4L80. He additionally mentioned he tows in "drive," not "overdrive." I'm not sure what that means. I think that's on "3?"

Are there a lot of mountains there? Altitude? Have you had this truck for a while? Or just bought it? Has it always had poor fuel range while towing? Unfortunately, I don't have much experience with TBIs, much less towing. Perhaps someone has a similar set-up towing similar weight in the similar road conditions. I've read upper 8MPG towing on Vortec, just have no idea on TBIs. We really need to know road conditions.

Thank goodness you did the conversion--I was starting to get a headache trying to do the translation in my head.......

4mpg is indeed pretty low. Should be more like 10-12 on relatively flat terrain, of course less at altitude in the hills.

My "light" K2500 with NV3500 and 3.73's pulled a 5K TT and would get around 10mpg at 6k to 9k in altitude in the mountains. It was third gear and hammer down up the hill though. In 4wd on dirt USFS or BLM roads and mileage was was like 6-7 mpg.

As Derek said the TBI's do best in the 2k-2.5K rpm range. By the time the motor hits 3500 rpms the swirl port heads plumb run out of breath.

The "standard" 2500 (not the light 2500 that was really a 1500 with the 14 bolt SF) came with the 14 bolt SF, the heavy duty options would bump the rear end up to the 14 bolt FF. I've seen a lot of true 2500's with the 14 bolt SF axle.

Boiling trans fluid ain't good, and at that point has probably already hurt the trans. The only factory HD coolers I've seen in GMT400's (which are 8 x 11 instead of the standard size 5 3/4 x 11) were in the 2500"s with the HD options and the 3500's.

The reason GM advised to tow in 3/D is so the TQ will lock up and would avoid extended runs with the TQ unlocked if driving in OD. That would make the trans boil over for sure..... On flat terrain while towing with 4.10's a guy could probably run in OD and the TQ would be locked most of the time.

Need to figure out why the trans is boiling over first.

 

[Supercharged111]

Thank goodness you did the conversion--I was starting to get a headache trying to do the translation in my head.......

4mpg is indeed pretty low. Should be more like 10-12 on relatively flat terrain, of course less at altitude in the hills.

My "light" K2500 with NV3500 and 3.73's pulled a 5K TT and would get around 10mpg at 6k to 9k in altitude in the mountains. It was third gear and hammer down up the hill though. In 4wd on dirt USFS or BLM roads and mileage was was like 6-7 mpg.

As Derek said the TBI's do best in the 2k-2.5K rpm range. By the time the motor hits 3500 rpms the swirl port heads plumb run out of breath.

The "standard" 2500 (not the light 2500 that was really a 1500 with the 14 bolt SF) came with the 14 bolt SF, the heavy duty options would bump the rear end up to the 14 bolt FF. I've seen a lot of true 2500's with the 14 bolt SF axle.

Boiling trans fluid ain't good, and at that point has probably already hurt the trans. The only factory HD coolers I've seen in GMT400's (which are 8 x 11 instead of the standard size 5 3/4 x 11) were in the 2500"s with the HD options and the 3500's.

The reason GM advised to tow in 3/D is so the TQ will lock up and would avoid extended runs with the TQ unlocked if driving in OD. That would make the trans boil over for sure..... On flat terrain while towing with 4.10's a guy could probably run in OD and the TQ would be locked most of the time.

Need to figure out why the trans is boiling over first.

That calculation makes assumptions based on needle movement and not actual gallons so don't get too excited.

 

[alpinecrick]

That calculation makes assumptions based on needle movement and not actual gallons so don't get too excited.

Well, I'm hoping the OP did the calculations based on the odometer and the gas pump reading........

 

[Supercharged111]

Well, I'm hoping the OP did the calculations based on the odometer and the gas pump reading........

When I read it, I didn't get that impression. Hopefully he can jump in and clarify.

 

[stutaeng]

True, that's based on level of tank based on instrument cluster, which is indeed not too accurate; just a "three quarter full," etc., as stated by the OP.

I don't know on the TBIs, but the vortec:

2500 with diesel and big block got the full float 14 bolt, otherwise it got the semifloat 14 bolt. Axle ratio could be 3.42, 3.73, or 4.10 depending on engine.

The 3500 vortec got full float 14 bolt regardless of engine. Axle ratio was 4.10 standard with 4.56 depending on engine and/cab.

 

[Supercharged111]

True, that's based on level of tank based on instrument cluster, which is indeed not too accurate; just a "three quarter full," etc., as stated by the OP.

I don't know on the TBIs, but the vortec:

2500 with diesel and big block got the full float 14 bolt, otherwise it got the semifloat 14 bolt. Axle ratio could be 3.42, 3.73, or 4.10 depending on engine.

The 3500 vortec got full float 14 bolt regardless of engine. Axle ratio was 4.10 standard with 4.56 depending on engine and/cab.

That's my understanding as well.

 

[stutaeng]

True, that's based on level of tank based on instrument cluster, which is indeed not too accurate; just a "three quarter full," etc., as stated by the OP.

I don't know on the TBIs, but the vortec:

2500 with diesel and big block got the full float 14 bolt, otherwise it got the semifloat 14 bolt. Axle ratio could be 3.42, 3.73, or 4.10 depending on engine.

The 3500 vortec got full float 14 bolt regardless of engine. Axle ratio was 4.10 standard with 4.56 depending on engine and/cab.

Actually, the C2500 is also listed with a 305!

BTW what that's a 305? LOL I've seen only one 2500 truck with 5.0 listed.

My brother and were talking and we were saying you hardly see any trucks with that engine. I told him almost all 305s have been remelted into LS engines, LOL.