What is the proper or excepted RPM range of a 83 300 SD at 60 mph w/14" (195 70 14)stock size tires? Will upping the tire size say to 225 50 16 bring it down? The engine revs at 3 (3,000) rpm at 65 mph...normal??
Thought it was rather high...BTW...it's a automatic

Normal ..it is the diff gearing on them.

But , some guys run 215/75/14 on the stock rims ..that gives you a 2" diam difference [ from 24.7 to 26.7]
The 255/50/16 will only give you a difference of1.3" and you will have to buy 16" rims besides..plus I think they are too fat for the chassis.

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A Dalton