Quote:
Originally Posted by jeff88
I've read snippets of info about taller gears, closely spaced, various features for a transmission, but I have yet to find a thread (or outside article) that concretely states what the best combination would be (at least economically speaking). So I'm starting this thread to hopefully try to hash out the best option.
Here's my best preliminary guess:
1st, 2nd and 3rd gears are all short and closely spaced. (This allows for quick acceleration, allowing the driver to get to speed as quickly as possible.)
4th and 5th gears begin to get taller and are spaced out more to allow efficient operation while at the lower end of the top speed (say, between 40MPH and 60MPH).
6th gear is extremely tall and will allow for most efficiency possible at a higher speed (say, 65 or 70+).
This of course assumes a 6 speed, but maybe there is a better amount? More gears? Less gears? Also, is there a reason why a manual and auto would require something different to maximize efficiency?
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Depends on the car, specifically the weight, the engine, the aerodynamics, the final drive, and the desired cruising speed. Thenorm was right on the money about needing to know the BSFC of your engine. The problem you run into is that most readily available manual transmissions you only have so much spread between your lowest and your highest gears and you have to compromise somewhere. For example my Toyota Celica has a c60 six speed manual, gearing as follows:
1 3.166
2 2.05
3 1.481
4 1.166
5 0.916
6 0.725
Final 4.529
The super low final drive makes takeoff quick and peppy, but it bites me when my crusing RPMs are high. I plan on regearing the final drive on my car to a 3.9 when the transaxle needs rebuilt, but I will lose the peppiness on takeoff to gain my better cruising rpms. If my car had 5th at .725 and 6th at 0.50 or if 1-4 were lower and I had a taller final drive would also be pretty ideal for my car, but in the world of just picking between drop in parts you have to make compromises . Different cars with different RPM ranges, power bands, weights, etcetera will require different transmission options. If I was starting with a blank sheet of paper and could have any transmission I wanted, I would get one with the widest range between first and last gears, and then adjust the final drive to suit the car and driving style.
The automatic transmissions have a torque converter, which basically acts like a lower gear on high torque loads, thus you can get away with a lot taller gears and still have some of the peppiness down low. The disadvantage is that automatics have more losses, so you are giving up HP to get that better gearing.