Boy I'm glad i saw this post...its something I've been wondering about. I run my tracker with stock tires and a 3 speed tranny 180 miles round trip 4 days a week and I run between 70 and 80 MPH, RPM's are around 4000-4500 and it seems to be about 3/4 throttle. Its a little sluggish but will stil accelerate at that speed.
Highest I have had mine is about 90 ish. And everyone else said it right--the wind does create havok on the interstate at high speeds....