well if he had put lower gears in it, it would get up faster.
It might accelerate faster, but the velocity will be slower.
Try to do this:Put it in 4th gear and cruise at 50MPH, 60MPH, and then 70MPH and record the RPM at those three speeds in 4th gear.
Then hopefully someone else is reading this and will do the same in their truck and will be able to do the same so you can compare RPM numbers with the vehicle's actual speed.
Regards,
BigMike