My buddy's got a '98 F250, 4.6L, Mazda 5-speed, and a 3.31 (or 3.3, something like that) rear end. The truck doesn't have a tach. I'm curious what RPM your trucks are turning out on the highway with similar set ups? A lotta times when towing a trailer up hills, he'll stuff it down into 3rd and run it up to 60-65mph. I guessed its probably turning around 4k under that load.

 

I have never done it, but I found out you can get some information through the dgital odometer. I don't know if 98 trucks had digital odo's, though.

Link.

 

I'm not totally sure but I think that the digital odometers were'nt 'till 1999. If so, that may not be an option.