RadioFlyer

I'll be the voice on the other side of the coin... I just ordered 3.9 gears for my MT, that's up from 3.69. I suspect it will make little difference, but while I'm in there installing an LSD, I figured I'd give the marginally shorter ratio a try. I have a Stillen supercharger on the car, and I dyno a bit over 400 at the wheels. I chose to do this for a few reasons:

1) When people are saying that they have no traction with the stock ratios, it's because of the open/viscus stock diff. Address the wheelspin, and you'll want more gear.

2) The stock ratios (with the stock ring/pinion) are geared really long and wide in my opinion. 2nd gear runs out to like, 70mph - that's too long for a sports car! And the gearing tops out at over 200, that's too long in my opinion. Dropping the FD ratio puts that more inline with other sports cars, and keeps you active on the gearbox, which I prefer.

3) It's not that much power. 400-450 isn't too much power as long as you're hooking up. It's about as much power as a modern Camaro/Mustang is putting down stock, and they're gearing down too. It would be a different story if I had more power though. With more power, I'd want to stick with the stock ratios, but 400-450 isn't THAT much. The BMW 550i does that stock, and it's a luxo sedan. And I believe they change the gear ratio for the M-Sport package. That's not even talking about the M5.

But in the end, it all might be moot because the difference is minimal between 3.69 and 3.91. Consider that for G37's, they use a 3.69 for the 6MT coupe, and a 3.9 for the 6MT convertible just to offset the weight. So it doesn't sound like there will be a whole slew of downsides to trying it out, and if you're already in there changing out the diff (and addressing my first point), then you may as well give it a try. Just my 2C.