Gear Ratio Change
 

I have the automatic FRS and plan on a rear end gear ratio change.
I want as much performance as possible but keep the engine below 3000 rpm's at 70 mph. I also will be going to a better tire, not decided yet but thinking 235/40/17.
Here's what I came up with, please check my figures and correct me if i'm wrong, and please offer any advice you might have.

Stock
tires-215/45/17, 24.6 dia.
6th gear tranny, .582
rear end, 4.10
RPM @ 70 mph, 2281

Proposed changes, option 1
tires-235/40/17, 24.4 dia
6th gear tranny, .582
new rear end gears, 4.80
RPM @ 70 mph, 2693

option 2,
same tires, but 5.10 rear end gears
RPM @ 70 mph, 2861

I'm thinking the 5.10 gears would be best.

Comments, suggestions please.

Are you going to track your car? The final gear also depends on what kind of track you will use it on. Shorter gears will cause you to shift more, and the AT has tall 5th and 6th gears, so it would be bad to be using 5th and 6th a lot on the track. The 5.1 is interesting, but for my local track I top out 4th gear after the longest straight. So if I use the 5.1, I would be using 5th gear then. Shifting gears all the time also slows you down, so I look at being in 2-3-4 gears at the local track I go to. I'm using a 4.55 FD. I'm interested in looking at the 4.8 though.

Yes, my car will be a track car/daily driver. My current track car has a 6 speed sequential gear box with paddle shift. 100 shifts to make a lap at Oregon Raceway Park, so I don't mind shifting. Unless there would be some adverse effect to using 5th and 6th on the track. My two favorite tracks, ORP and Chuckwalla Valley, have 1/4 mile straights.

The big question is, is road RPM more important, or gearing properly for the track more important?

All of these numbers will be off if/when you make further changes to the car.

I don't think that is right.... stock config @ 70 mph should be turning just a bit above 3000 rpm...

Anyone mind providing a quick down 'n' dirty explanation on gear ratios? Maybe not here to keep the thread on point with OP's concern, but maybe its own thread.


My BRZ is the first car I'm actually modifying so this forum has become a wealth of knowledge. But proper gearing ratios is something I only ever see a few informed people conversing about on here. The way I see it either I'm the only willing to ask or everyone else already knows, and my money is on the former.

Shorter gears = more shifting, but more mechanical torque multiplication.

Longer gears = less shifting, but less mechanical multiplication.

It's like comparing 3rd gear power to 4th gear power, except ALL your gears are getting "lower (shorter)" with a higher final drive.

The price is your RPMs are always higher for street driving.

I just came back from town and 70 mph in cruize control was tacking just under 2300 rpm.

Proper gearing for the track is more important.

How big of a job is this/how reliable is it? I stumbled across this topic and had the thought that I could do the same thing to my manual trans for its final drive. I drive 90% on the highway at about 120km/h and hover around 32-3400 rpm which seems high over the long term. Ideally it would be under 3 imo. Any thoughts or opinions on that? Thanks

Oh whoops, automatic, duh

Will you ever go forced induction in the future? Are you going to be investing in a transmission cooler?

More shifting means more heat in your transmission (AT).

it just seems to me that its one of those high dollar/low gains kind of things that should be pretty low on your list of priorities. if you are concerned with highway driving at all in this car, it probably shouldnt be considered. dump that money in to suspension or tires or power or brakes etc and shed more time while still having a mild cruising rpm.

Yes I will be adding a tranny cooler, oil cooler, and with time wider tires, brakes, coil overs, and yes, some kind of forced induction.