Yep, those ranges for 6 or 7 tooth drive gear is what came stock. If he originally had something like a 3.30 rearend (7 tooth) and now has 3.70 rearend, did the PO change the drive gear to a 6? Who knows. That is good info if everything is stock.

Instead of the math, you can always do the trial and error route with different driven gears and a GPS. That is a sure fire way to get it right. If you cannot get a driven gear with enough teeth to work, you may need to change out the drive gear.

More than likely either a 17 or a 20 tooth driven gear will work for you depending on what drive gear you have.

 

as for the trial and error method
if you don't have a atnav or gps showing you the speed, let me know an di'll give you a table what exact speed your driving at which rpm