ISM or something else.

I would agree, go with a bigger motor as you would not work it so hard. When i was hauling corrogated boxes which we would gross out to low 50,000 lbs. Our first tractors were 98-99 int.9100 and had series 60 Detroit's in them and they were single axle's. Some days we could get up to 8 mpg, some days around 6 mph but never under 6mph,5-6 years later all the new trucks we got were Int. 8600's with the ISM cummins and we had problems with pulling power we could never get even 6mph it was all in the mid 5's, in winter we would get mid to high 4mph. The Detroit's could have been the 12.7 i know that they were a big motor for a single axle.

 

I drive a tractor with an ISM 320 and 13 sp and it just can't pull hills.I usually get 6.8 mpg with it.

 

The ISM we had were 300 hp and 10 speeds, maybe it was on how they had the motor set up, we could also not pull hills but still could not get good mileage, i could get 6 mpg bobtailing, pathetic ain't it.

 

All depends on how the ISM is rated, I am usually right at 45,000 to 50,000 and you can call BS if you want, but I am always at or close to 8.5 to 9 mpg, BUT this is with a 425hp 1550tq ISM so I have the power, but I also get the mileage of a smaller motor.

If you can find an ISM anywhere from 370 to 425 I would think you would do fine, but forget about those 285/330/355 motors.

JMHO

 

The ISM isn't a slouch as long as you get like KANSAS TRANSIT said and get a 370 and up. Anything lower was intended on local daycabs or barebones fleet trucks. Though you can flash the ECM and uprate the HP. Cummins produced a 500hp version of the ISM that was sold to the chassis builders (RV's, buses, fire trucks). I don't think they ever offered it to the truck builders, I guess they figured if you wanted 500hp, you might as well step up to the ISX.

 

i have an 8600 with a 385 ism pulling flatbeds, its no slouch.

 

ISM's are great little motors, they run well, are trouble free, anyone can work on them, and you can just about buy parts for them at Wal-Mart!

The only bad press they got was when companies were trying to save money and speced them in trucks that were running over 60,000 and then trying to pull Eishenhower.

 

I've been driving a '98 M11 since 2007. Had some troubles early on (seller sold us a bill of goods) but after a couple of blown head gaskets, a couple of cylinder heads assy's everything has been pretty good. Finally found a mechanic that realized it was better to replace the sunken liner that keep putting heads on it. Not the engines fault. Bad mechanics.

I used to average 7 but more recently I'm down to 6.2. It's due to have the lash set and if that doesn't get my mileage back, then I blame the the crappy new fuel.

Worst part of the engine is the oil leaks.

I gross 80,000 south from Watertown to Baltimore and 60,000 north to Toronto.
370/410 hp
1998 T800
13 OD
22.5" rubber
62 mph no cruise
1750 rpm
4.33 rear gear

It's geared for ~55 mph. I bet I'd get real good fuel mileage there. If I drove 70 I'd only get 5 mpg