CaptCameltoe

iTrader: (3)

I don't have the 8.1, have the 6.0 and its in a 4wd ec dually. It does have 4.10's but avg mpg empty is 13-15 and towin 6k lbs or so is about 10. I rarely drive over 55-60 mph and the roads around here are all mountain for the most part which I'm sure hurts mpg some. its bone stock at the moment and honestly gas mileage is better then expected as far as I'm concerned. its a 4wd 1 ton, anything over 12 empty makes me smile

Fast355

Won't save ANY gas being in 4th compared to 3rd with that Undergeared beast. Likely the only place it will upshift into 4th will be on a downhill run and all the shifting will do is heat up the transmission and wear out your clutch packs.

I have 3.73s and tow a 6,000 lbs travel trailer with my Express vans small block 5.7. I have towed in between 55 and 75 mph and tend to get better mileage in the 70-75 mph range, with the engine running closer to peak torque. Anything under 2,500 rpm, FORGET about MPG under load.

Even the old 1994 Dodge 2500 V10 we had got better mpg running in 3rd than 4th (3.55 gears) when towing and it had ~500 ft/lbs of torque.

Oobray

9k at 80mph is a lot different than 5,200 lbs at 70. But my original question was also an attempt to "reorient" myself. Meaning I was trying to figure out if pulling a 6% grade with 5,200lbs and being completely maxed at 3,500rpm and about 55-58mph was "normal" for the 8.1.

Fast355

If you are over 5,000 ft elevation it is possible that it is giving all it has in stock form. That being said I am towing 6,000 lbs with a rig that is grossing over 12,000 lbs with the little old 5.7 Vortec. I have the same 4L80E transmission and 14-bolt you have with the same 3.73 gears. The 350 is not stock but I feel you still have more torque in the low-midrange than I do. I tow in 3rd gear and let it downshift to 2nd on its own with Tow/Haul. At 65 mph I let it eat until about 3/4 throttle before the shift tables force a downshift. At about 4,000-4,500 rpm the little 350 has no issues holding speed on long grades.

This is my tow rig and trailer


I will admit the 5.7 moves my Express better than someone would expect. Its basically a 300 HP marine 350 with some headwork, ported intake manifold, and headers in place of the marine manifolds or a Ramjet 350 with headwork and a ported marine intake. Probably makes 340 HP and 420 ft/lbs. Unloaded it has some punch for something that weighs 6,000 lbs and has a small block/4L80E driveline.

Vindii

This is nowhere near true. You will for sure get better gas mileage towing in 4th than in third. It will shift into third when its needs to. When it doesn't it will stay in forth and get better mileage while its in forth VS third due to lower RPM. Your trans will be fine behind a 8.1. Maybe not so much with the shifting it will do with the 350 though.

Yes my truck get better mileage at 80 than 65 (while towing). I suspect being closer to peak torque allows it to downshift to 3rd less often. Unloaded I dont see much difference from 80 to 65.

Maybe you guys used to own a 8.1 but its not the same as your 350 or 6.0.

Fast355

You have to take into account more than RPM. You are putting about 25-30% more load on the engine in overdrive which requires you to open the throttle more in overdrive and if it happens to be high enough load and throttle opening to go into PE mode or Catalyst Overheat/Piston Protect mode it will avsolutely DRINK fuel. With the stock tune in my 350 just running 70 mph into a headwind was enough to bring in Catalyst Overheat Fuel Enrichment about 1/2 the trip. Air/Fuel ratio was in the high 10s/low 11s on the stock tune. 8.1 has similar programming.

Vindii

Your comparing the experience with your 350 to a big block 496. It not the same. I am speaking from actual towing experience with the exact same truck the OP has.

It not going to stay in 4th. It will shift when it needs to pull a hill. Otherwise on flats and downhill it will cruise along in 4th.

Fast355

I tuned an 8.1/Allison Workhorse RV with the Banks setup on it and it liked running in the 2,500-4,000 rpm range under load and was geared accordingly.

Vindii

My truck runs 2300 rpm at 80 in 4th. Maybe 2100 at 70. Jumps to 3300 when it kicks down to third. Dont know that I'd want to run it up to 4K often with a load on it. Same 3:73 gears.

Fast355

Moded 350 vs 496, he is making MAYBE 30-40 ft/lbs more torque than my 350. 3.73s + .75 od is a 2.80 final drive. Big block or not its not going to tow very well with a 2.80 final drive.