try gear vendors they have a ,lot of info on t18s t19s my ford f250 with t18 trans and dana 60 3.73 locker did the same thing the rpms seemed high i got rid of the 30.975-16.5s and went to 225-75-16s it changed the final gear ratio to roughly 3.82 it helped a lot .f you go to 4lo.com it has calculators to figure out everything you can also download it to your favorites on your computer