Just get a GPS and check your MPH with it. Most will show MPH and just check your tach to know how fast you are going at a given rpm. If you still have the 5.14 gears then you will be pulling high rpm at a relatively slow speed. There are many different calculators to check this out. I personally would put an overdrive in if I was not going to change the rear gears. The .7 OD in a 700R4 would effectively change your final drive ratio from 5.14 to 3.6 which would be better than changing to 4.10 gears with the original 4 speed standard or your turbo 350 which would then be a final drive of 4.10 because they are both 1:1 final gears.
