Difference between 4 speed, vs 3 speed with overdrive transmission?
Just wondering what exactly the difference is between a 4 speed transmission and a 3 speed transmission with overdrive. What the heck is overdrive?
This is in reference to manual transmission. I know I have overdrive on my automatic truck, which I have never ever used, but was told it can be helpful for towing uphill at slow speeds? I'm more interested in street rod application however, as I'm looking at buying a toploader transmission for a ford galaxie… I will probably be going with a plain 3 speed as they seem to be the least expensive, but am curious as to what "overdrive" means in these transmissions…
1+1*2=?
When they originally came out 3 speed transmissions had a higher gear only to be used a highway speeds. It's geared much higher than a 4th gear. I've never heard of a 4 speed with overdrive, but it would be the same concept.
Overdrive is a higher gear ratio, which improves fuel economy when driving at highway speeds.
Overdrive is when the ratio is lower than 1:1. It helps with fuel economy but performance suffers.
The only general difference is a 4 speed has one additional gear compared to a 3 speed. But the transmission gear ratios can vary wildly between transmission affecting both performance and economy.
You turn the overdrive OFF when towing.
High gear in non-overdrive transmissions was always 1:1 ratio.
The engine speed and the output of the manual transmission is the same.
An automatic transmission used to have a few percent slippage due to the torque converter.
As an example, a 3-speed transmission might have these gear ratios:
1st - 2.5:1
2nd - 1.5:1
3rd - 1:1
So an overdrive gear might be something like.8:1
That's point eight to one ratio.
So the output of the transmission will be turning faster than the engine.
Or equivalently, the engine will be turning slower than the output of the transmission.
Effectively, at highway and cruising speed, the engine goes slower, and if the engine has enough torque at that rpm, there will be a fuel savings. The engine will also get less wear.
The best way to "see" the difference is to make a graph of engine rpm versus car speed. With known gear ratios and a known rear axle ratio, with a known tire diameter/circumference, a graph can be made. It's not so easy to show here.
I WANT TO REPLAECE A 3 SPEED MANUAL TOPLOADER WITH A 4 SPEED MANUAL IN MY 66 MUSTANG COUPE, WHAT IS NEEDED FOR THIS ARRANGEMENT
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