K3500 Gearing

[L31MaxExpress]

He said 2500 RPM at 63mph. I wanna say mine hits 2000 around 57. I have 4.10 and 225/75/16.

IMO that is perfect for towing a travel trailer. Less gear is going to make him get into the throttle more. Long period of open throttle will put the thing into PE. Say BYE to fuel mileage in PE.

I get 11 mpg towing at 65 mph with the 383 and 5.13s on LT265/75R16s. The 383 makes substantially more torque than a factory 454 even down at 2,500 rpm. Less rear gear and I would be downshifting for every hill.

I have towed the trailer with the Tahoe. 8.1/4L80E/3.42 gear and P305/50R20s. I lock the shifter into 3rd and roll. Even with the 8.1s torque the final OD ratio is too tall and it bogs the 8.1.

 

[Supercharged111]

Stock 454 ain't that great until you drop the gloves. With the blower, my empty hwy mpg remains at 10.7ish. Last fall with the massive truck camper in the bed we pointed the rig uphill, went from 6000' to over 9000' locked in OD the whole time grunting along. I know we hit PE in OD regularly. Still though, mpg only dipped to 9.7. Add the trailer though and it gets ugly. He'll He'll trailer alone seems to be the key to murdering fuel range.

 

[L31MaxExpress]

Stock 454 ain't that great until you drop the gloves. With the blower, my empty hwy mpg remains at 10.7ish. Last fall with the massive truck camper in the bed we pointed the rig uphill, went from 6000' to over 9000' locked in OD the whole time grunting along. I know we hit PE in OD regularly. Still though, mpg only dipped to 9.7. Add the trailer though and it gets ugly. He'll He'll trailer alone seems to be the key to murdering fuel range.

I agree...My trailer is about like pulling a parachute. 8' wide and 10' tall. That is 80 square feet of frontal area that is not all that aerodynamic. My Tahoe dropped from 18 mpg I get running 75-85 mph down to 10-11 pulling it along at 65 mph in 3rd gear. Even in 3rd gear I was not in lean cruise even on the flats. Running along at 85 mph the 8.1 is only turning 2,500 rpm in OD and stays in lean cruise even uphill. Climbing Routan pass I was in 3rd gear at 60 mph at 2,400 rpm and about 50% throttle opening. Cammed 8.1 had plenty of power to go faster, just did not want to eat all that extra fuel.

 

[Pinger]

What is PE?

 

[L31MaxExpress]

What is PE?

Power Enrichment...Basically your PCM dumping extra fuel like crazy to cool the pistons and cats.

 

[andy396]

Power Enrichment...Basically your PCM dumping extra fuel like crazy to cool the pistons and cats.

While on the subject of PE, what’s the theory behind a more rich fuel mixture cooling the cats? Wouldn’t it produce more emissions and make the cats work harder?

 

[Pinger]

Power Enrichment...Basically your PCM dumping extra fuel like crazy to cool the pistons and cats.

This is how they are set up from the factory?

 

[Supercharged111]

While on the subject of PE, what’s the theory behind a more rich fuel mixture cooling the cats? Wouldn’t it produce more emissions and make the cats work harder?

Cats can't keep up with PE fueling, no way around that. When they get real hot, the PCM dumps even more fuel to save the cats. When coolant is stupid hot it'll dump even more to cool the Pistons. Regular PE is only for performance.

 

[L31MaxExpress]

Cats can't keep up with PE fueling, no way around that. When they get real hot, the PCM dumps even more fuel to save the cats. When coolant is stupid hot it'll dump even more to cool the Pistons. Regular PE is only for performance.

Regular PE is for performance but also for piston cooling and choking the oxygen out of the exhaust stream for the cats as well.

 

[andy396]

Cats can't keep up with PE fueling, no way around that. When they get real hot, the PCM dumps even more fuel to save the cats. When coolant is stupid hot it'll dump even more to cool the Pistons. Regular PE is only for performance.

I still don’t understand GM’s logic behind setting the delay on PE to 60 seconds and then when PE does come in it dumps fuel at 11.5:1 afr which is way below optimal power afr. It makes no sense, but surely they have their reasons. It’s not like the optimal afr for power wasn’t figured way before the 1990’s.