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Adding Overdrive

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Dave:
When you run third O/D are you running slighly less revs than you would run at the same speed in 4th non O/D?
 
Slightly more revs.

Third OD turns the engine 113 rpm faster that fourth direct at 60 mph. Practically no difference.

My third gear ratio is 1.33 & the OD is 28%. Overall ratio in third over is 1.05 vs 1.0 in fourth direct.
D
 
Hi again,

Per the factory manual, here are the final drive ratios with a 22% A-type on a stock TR4/4A gearbox, and with the two most common crown wheel & pinion combinations....

3.7:1 CW&P:

1st gear - 11.93
2nd gear - 7.44
2nd OD - 6.1
3rd gear - 4.9
3rd OD - 4.02
4th gear - 3.7
4th OD - 3.034

4.1:1 CW&P:

1st gear - 12.87
2nd gear - 8.24
2nd OD - 6.76
3rd gear - 5.44
3rd OD - 4.46
4th gear - 4.1
4th OD - 3.36


I agree that the primary use for any OD would be top gear on the highway. But particularly with a 4-cylinder engine, it sure will be nice to have opportunity to select just the right gear in a variety of driving situations.

Cheers!

Alan
 
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