Tab,
With OD you should get about 22 mph per 1000 rpm. 80 / 22 = 3640 rpm.
I take my '72 GT to Annapolis from New Haven CT occasionally. Running
at 3000 rpm gives a cruise speed of 66 mph. On this trip the car always
gets 33+ mpg.
However, I once had a '75, non OD car. With its single ZS, it also got
33 mph on the same trips, though that was ten years ago when most people
drove slower.
Gas mileage is primarily a function of wind resistance. There isn't
much power required to roll a car -- it's all in pushing through the
wind. That said, if it takes 20 hp to shove the car at 60 mph, it's gonna
take 20 hp whether it has OD or not, and the engine has to produce that
power. Since the engine is likely to be more efficient at 3000 rpm than
at 4200, there may be an increase in MPG, but it's not going to be huge.
It's not going to justify the cost of the OD, ever.
But, since high engine speed causes a certain amount of wear and tear
on you, it still might be worth having. I certainly enjoy having it.
This bit about flicking from 3 to 3OD around town is fun for a while, but
it's not all that useful aside from the Gee Whiz factor. Where the OD is
great is on the highway.
Bob
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