Dumb transmission question

The 3406B is happiest at 1600 to 1900
The 3.70 on 24.5 tall rubber will give you close to 92 at 1900 or so.
22.5 tall (or24.5 l.p.) will give you 85 at 1900.
A 3406B will be a real fuel hog at 1900 for long stretches they are most driveable and "somewhat economical" at 1650 or 1700.
Also add 1 to 2 percent for tire growth at anything over 70 mph and I suggest that you never run a tall tire recap over 75

 

Go with an Rtlo18918b. Little difference in price and more plentiful and newer than the smaller ones. That is what I put in behind my 3zj mechanical in place of the original 15spd. 3.70 on tall 24.5 will clear 100mph if that is in the back of your mind. Not condoning that at all btw. Fyi, 3.70's on tall 24.5 are almost identical to 3.55's on lo pro 24.5. Spent alot of time and money on my mechanical to make it strong, steady and dependable but the 18spd was the best money I ever spent to improve performance.