To me anyway....

I was reading up on the technical specs of our cars. I noticed that the 07 sedans have the same final drive ratio for the auto's and the 6mt's. But then when I looked at the 07 coupe, the auto had a final drive of 3.36, and the 6mt was 3.54. I then checked for the g37 and it was also the same for the auto and the 6mt. 3.69 for both the g37 and our 07 sedans.

I also was reading that increasing the tire/rim size on your car will lower your gearing. So basicly with me having 20's on the car and it having 3.69 from the factory, what exactly would that put me at?

My curiosty on the subject was actually brought about from playing forza 2 for xbox 360 lol..(dont laugh). The game is actually very precise for real world performance so I was playing with different parts and noticing the changes.

What exactly is entailed with changing things in the car such as the final drive ratio (and how much). Is it changing the internals to the transmission, or actually the entire transmission itself?

Im hoping trey chimes in with an answer for this.

If people are wondering why I would wanna do this...The low end acceleration on this car sometimes feels lacking to me. I ntoiced cars like the IS350, which supposidly are alot faster from the dig then our cars have a final drive ratio of 4.01, which explains why they die down up top.

 

IIRC, final drive can be changed with gearing in the rear diff. And low gearing won't make a car die up-top (unless you mean reduce max mph), it will just hit redline faster. If the IS350s have a problem at the top of the powerband, it's an engine thing.

 

Going to an overall larger rim/tire size will affect your FD ratio. However if you decide to swap out your stock wheels for a set of aftermarket 20s, you'll obviously have to buy new tires as well. These new tires will have a smaller sidewall to compensate for a larger diameter rim effectively keeping the overall rim/tire diameter the same. This is known as plus sizing. If you were downsizing to a 16" wheel, you'd need a tire with a larger sidewall to compensate for the smaller diameter rim. Basically your wheel diameter changes as does your tire sidewall height to compensate, but the wheel/tire combo diameter does not change at all, otherwise like you said, it'd through your FD and your speedometer off.

 

when i say "up-top" i dont mean in the rpm band. What i mean is the higher final drive ratio yields a better lets call it 0-80 but after 80 will start to die down.

 

Still shouldn't matter - unless you drive the final ratio to something REALLY radical, it'll still easily pull past 120 - and get there faster. The only thing final drive will do (other than decrease gas mileage) is decrease the maximum top speed - and that's only if the car hits top speed at redline. If it's a) it's already electronically limited, or b) it's drag limited, there may not be a decrease.