Clearly the army wanted to go let the deuce go faster than 45, or they wouldn't have gone to the trouble of modifying a big heavy diesel engine to go above 2200RPM, and they wouldn't have modified the gasser's transmission to have an overdrive 4th gear when used with the MF engine.-Chuck
So this is an interesting topic, at least for me.
The overdrive 4th (and yes, this is indeed correct, the 4th gear became the OD gear, explaining the shift pattern!) is mostly, IMO, a compensation for the extraordinarily low axle gear ratio (6.72).
The axles have a low ratio to keep driveline torque down (with cascading design needs and tolerances back through the transfer case, transmission, clutch, and engine), and the axles are also double reduction, allowing for a very high (and thus more protected) driveshaft, with excellent ujoint angles.
So, yes, the Overdrive gear is obviously included for higher road speed, but why it's there brings up an interesting issue overall with running these vehicles at speeds higher than convoy speed, which is driveline speed and its associated implications (engine RPM, transmission gearing, transfer case lubrication, tire size and also, theoretically, critical driveshaft speed). IMO, all of these need to be thought of when running these vehicles stock at higher speeds, and especially even more when a single component out of this group is changed.
I love talking about these trucks. They are great platforms, but there are definitely issues with them that need addressing if they are to have good-to-excellent high speed road manners.
Best,
Bob
