I don't often jump into these discussions, however I think I will join this
one. Bob Howard and Lew Palmer are absolutely correct. An overdrive is either
an add on unit or a gear ratio in the gear box, achieved with extra gears (ie. 5
speed or 6 speed) that gives a final ratio lower than 1:1 at the input to the
rear axle. A lowered gear ratio in the rear axle is just that, a lower rear
axle gear ratio and affects the MPH/REV in each of the gear box's position.
There are trucks in use (or used to be) that have two speed rear axles that are
controlled from the gear shift lever, but they are termed just that, two speed
rear axle as opposed to overdrive.
Not to get into an argument with Lew, but I believe that the statement "The
second problem is that with a 1:1 ration in top gear, the engine is running at
much too high an RPM for comfortable cruising at 60 - 65 MPH" is a subjective
assessment, based on the American preference for large displacement low revving
engines. My wife and I have twice taken our TD, with standard gearing on 5000+
mile trips in the Western USA, traveling 8 to 10 hrs a day at 4200 RPM and the
worst problem that we had was a fouled plug at the end of the last trip. The
high RPM did not in any way damage the engine (other than to wear it faster),
although I do admit to wishing for an overdrive just to make ME feel better.
Hope that this doesn't muddy the waters too much.
Sincerely,
Dave DuBois
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