Short answer is probably about 85-90 mph on the flat with no wind and no load, and no modifications.
But there are a lot of qualifications to this. Starting problem would be if the engine is governed, which nearly all trucks are. School buses, for instance, in Virginia, are governed so that they can't go faster than 55 mph. Schneider, a big trucking company, I think governs their trucks at 70 mph.
Then you have gearing. Although cars have gearing, truckers actually think the process through. Gearing depends on terrain where you operate and fuel efficiency. You might not want to gear a truck out to where it could do 90 because of losing some power in more useful ranges.
After that look at the load. Even on the flat, load effects speed, especially on accelerating, but throw in hills and the equation becomes even more interesting.
Next problem is fuel. You're probably looking at about 6 mpg at 70 mph. Bring that up to 90 mph and you'll be drinking fuel so fast it will make your gas guage spin. And even without the recent price hikes in diesel, truckers are aware of fuel costs. Depending on your tanks, a truck can take from $200 to over $500 of fuel. And depending on what type of driving you're doing, you can be filling up every day.
Real world is most trucks max out at around 80 mph, and these will be trucks who predominately run west of the Mississippi. Pretty much what you see on the highway is their maximum speed.
Best of luck,
Jim Clark-Dawe
