Forgot to give an example of the relationship between transmission and rear differential.
Say you've got 28" tires, the transmission ratio is 3.76:1 in 1st and 2.18 in second, and the rear end is 4.11:1 and you want to know how fast you'll go before you have to shift gears. If you know your powerband is between 2,220 and 4,000, using algebra or an online calculator (
http://ultimategto.com/art29.htm or
http://www.csgnetwork.com/multirpmcalc.html you'll figure out that:
In first gear when you get to about 20mph you'll be turning almost 4,000 rpms have to shift to second.
With the .72 overdrive at 65 MPH you'll be at 2,300 rpms and just at the beginning of the power band, however at 80 you'll only be up to 2,800 RPMs, have power to spare to get up hills and not be burning too much fuel. If you are anxious to get up to 75mph (or want to downshift and get some power to pass) you can safely stay in fourth (at 1:1) unitl 80 without breaking 4,000 RPMs!
If you had lower gears, say 2.73:1, in 5th gear at 80 mph you'd be at only 1,800 RPMs and have to down shift to pass or get up hills. So, 2.73 is too high of a ratio.
Basically, you have to multiply the rear differential ratio by the final transmission ratio. So, if the transmission is .72 and the Diff is 3.73, it'll take 2.7 revolutions of the engine to get the tires to go around once. With a 28" tire that means 2,578 revolutions to travel at 80 MPH.
