3.90:1 Rear gear ratio
 

Has anyone done this? I'd like lower RPM on the highway and for when I take the FI dive more spread between gears. Thanks I did a search and didn't see that anyone had done this. These differentials are out of 99' up Miatas.

Yes high hp setups have done the swap. It would suck NA but if turbo and running 500+ would probably be good.

^ yea alot easier to run with the vettes at 140+.

I believe Sideways is running this or similar. Probably for the reason ChefJ suggests. ;)

I put a Miata 3.9 into my race car. Should have gone for the 3.63 as I ran out of revs on the 3.9.

Drops straight in without modification.

Anyone know what RPM @ 60 is with these gears? Or the top speed in 6th is with stock tires?

Top speed remains the same as it is with the stock 4.10 with a stock engine.

Wouldn't a 3.9 FD in the s2k effectively make the gears about 5% taller? It would be too small a difference to be worth it imo. If you went for say something like a 3.7, you'd make your gearing about 10% taller. So, instead of cruising in 6th at 65mph at 3500rpm, you'd be cruising at about 3150rpms at 65mph, or just under 3krpm at 60mph. This would certainly make cruising at 75+mph on the highway much more bearable, and actually help gas mileage.. The only other question is whether there's a "fix" for the speedo now reading slow..

The thought has crossed my mind about putting in a taller FD, to make highway cruising more bearable.. Anyone who's done a taller FD in an AP2 care to comment? People who are saying that a taller FD would really suck NA, I don't quite follow. Yes, it changes the nature of the car a little to be a little more docile, but seems not terribly different for folks familiar w/ b-series trannies back in the day going from say an ITR to GSR tranny. I just wish honda threw a taller 6th in for cruising.

i have a set of 3.91 gears and enjoy them. My speedo is off by about 3-5mph depending on what tire/wheel selection i am using. I enjoy the gears because with a ap1 transmission and ap2 motor, i make up for the loss in RPMs per gear due to my 8500 rpm redline. The difference isn't night and day, but i enjoy them. You can use a yellowjacket to modify the speedo if you have issues. It can be more annoying in regards to law enforcement with longer gearing than short because you are actually going faster than what your speedo is saying, instead of the other way around...

To my knowledge, the modifry yellow jacket only compensates for larger FD's, not smaller, by allowing someone to adjust the speedo lower, not higher. If there's something else that would work, that'd be good to know.

Soul coughing, may I ask what kind of FD you got? I was looking up info about swapping FD's, but didn't seem to find too much info about swapping smaller FD's for s2k's, only larger ones.

Edit: Ok, I guess there's also a "yellow box" that coudl probably work.