ASM is an option in the "Driving Options" menu, found next to the setup menu in the pre-race screen.
It does sound like your problem is coming from the transmission though. Make sure you have the full custom transmission installed, and click on the button on the right side of it to bring up the window that has all the gear ratios. Then you can adjust the Final Gear by moving the slider -- I think to the left -- until the max speed number at the top right of the window goes up to what you want it to be(it's usually a little higher than the speed you'll actually reach). You can also just change the Top Speed setting at the bottom which some people prefer. If you screw something up there is a button to go back to the defaults, so don't worry too much.
Alternatively, you could probably try installing the stock transmission for tracks that fast since I think most of the trucks have extremely high gearing originally. It won't be as efficient but should remove the top speed limit.