Originally Posted by kenfoozed
I am looking for a RE 40' bus with either a Cummins 8.3 or DT466, backed with a MT634 and "highway gears"-- still need to figure that ratio out.
This is a common dilemma.
So it's not "one thing" that makes the bus good on the highway, it's the power-train combo.
Most of the gearboxes do not have overdrive gears. That includes both the AT545 and MT643.
You are could easily find an AT545 behind the DT466, unless it's tuned for higher power. You will not find that transmission behind the Cummins 8.3 because the engine is too powerful (usually 250 hp in a school bus).
You are most likely to find either the MT643 or the MD3060. EIther works well, but the transmission has implications for the Final Drive Ratio.
The MD3060 has a 0.75 OD (5th), and the possibility of 0.65 (6th). With that transmission a FD ratio of 5.38, even 6.14 is perfectly practical.
Those numbers behind an MT643 would be too high. With that trans you would want an FD ratio somewhere in the 4's to give adequate cruising speeds at lower revs.
So when you are looking for a bus, it's the combination that gets you what you want, not any individual element.