Final Gear Ratio??

[losispeed]

Noob question... What does the final gear ratio setting do? It does not seem to affect auto settings or any other settings... Thanks for your help...
-The Noob.....

 

[VG30DETT]

It's the gear ratio in your differential. It affects all the gears, for example:

The Nissan Sentra SE-R Spec V has a six speed manual transmission. The gear ratios are as follows:

1st: 3.417:1
2nd: 1.944:1
3rd: 1.258:1
4th: 0.947:1
5th: 0.773:1
6th: 0.630:1

Final Drive: 4.429:1

When power is sent through the gear box, it has a ratio of 3.417:1. The 3.417 is the ratio of the gears in the output shaft (going towards the wheels) to the ratio of the input shaft (coming from the engine). Then the power from that is sent to the differential, where this a 4.429:1 gear ratio. This gives you a total of a 15.134 (3.417 x 4.429 = 15.134) ratio to the wheels, from the engine. As far as top speed and acceleration go, a bigger number as the final drive will have you accelerate a lot faster, but a lower top speed. A smaller number will have the adverse effect.

Using the Nissan as an example still, the 180ft-lbs of torque it's pushing out isn't really whats being created at the ground. This 180 ft-lbs of torque is being multiplied by all of those gears. So in first gear, you actually putting closer to 2724 ft-lbs of torque to the ground in first gear. Take away some of the torque for the loses in the drivetrain and you get less than that number.

I think I explained all of that right.... Don't hold it as 100% truth until others verify. Also, I happened to have a Sport Compact Car that gave me all of the gear ratios and torque numbers, etc.

 

[losispeed]

Thanks that was very well put.

 

[Duke]

Originally posted by VG30DETT
Using the Nissan as an example still, the 180ft-lbs of torque it's pushing out isn't really whats being created at the ground. This 180 ft-lbs of torque is being multiplied by all of those gears. So in first gear, you actually putting closer to 2724 ft-lbs of torque to the ground in first gear.

I think I explained all of that right.... Don't hold it as 100% truth until others verify.

Actually, you got it mostly right. Just a few corrections:

1) The RPM of the output shaft is actually divided, not multiplied, by the final drive.

2) Torque isn't actually multiplied by the gear ratios. If it was, you'd leave the car in top gear all the time and enjoy huge acceleration power. The torque required to accelerate the car is multiplied by the effective gear ratio, but the torque available from the engine remains constant. That's why there are som many different gears in a transmission.

 

[VG30DETT]

Ahhh, drat. I knew I couldn't get it 100% right. Thanks!
About the gear things, thats what I meant, but I r dumbz0r and couldn't explain it correctly.