Difference between 4spd and 3spd+overdrive?

[Scott Feit]

Is overdrive really different than just another gear? Or is it just some kind of marketing term that doesn’t really mean anything?

Topic

[dave]

I like overdrive because it is fuel efficient.

Overdrive means the transmission output actually spins faster than the engine input.

The most you can ever get out of a non-overdrive transmission is a 1:1 “direct drive” ratio… where the transmission input and output are locked together and spin at the same rate.

Technical Mumbo Jumbo

• In automatic transmissions, overdrive is accomplished by holding the sun gear, driving the planetary carrier, and the ring gear is the output.
• In manual transmissions, overdrive is accomplished by driving a counter-shaft “cluster” gear that is larger than the output shaft speed gear (usually 5th gear, with 4th gear being a ‘direct drive.’)

In these days of rising gas prices, go for that overdrive transmission.

[Scott Feit]

Thanks for the response, but I’m curious as to why it’s called an “overdrive” transmission when something like ZF’s 8 speed also has ratios greater than 1:! (both 7th and 8th gear in fact). Does “overdrive” really mean anything special, or is it just what they decided to call trannies with ratios > 1:1 back when such a thing was new, to differentiate them from other trannies (i.e. marketing purposes)? Is the ZF 8HP technically a “dual overdrive” or some such thing?

[dave]

In my opinion, any transmission that can demonstrate the property of producing an output result that is greater than it’s input will achieve ‘overdrive’ status, regardless of the number of overdrive gears.

[Author]

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