Isx detuned?

[Chamberpains]

The way you describe your RPMs at 1800 doing 65 to 70 miles an hour tells me your rear end gears are 4:10's or better. That does not sound like a Walmart fleet truck at all. They gear their trucks with crazy low ratios in the rear. It sounds like you may have got your hands on someone that leased their truck to Walmart. That thing should be a pulling beast going up hills.

What you could also be describing is a torque limiter. I know on my 06 ISX I can set the heavy and light torque limiter at 1750 RPMs and it will fall flat on its face exactly at that RPM. you could also have the gear down protection enabled. There are a couple different torque limiters on your engine. And it wouldn't be unlike Walmart to set their trucks up to have all of them. You may want to invest in Cummins insite to check and change these governors if they're set. Also you'll be able to see if it's silently derating or going into an active derate under acceleration. it's well worth the money to invest in it so you can watch what's going on and keep your money in your pocket. It's a relatively simple program to figure out especially on your engine. And will help you diagnose the many many many other problems that may arise.