4.08 or 3.90 gear questions...
 

I'm just wondering how much power do you lose up top? What speed does it stop pulling hard? Do the 3.90's really make THAT much of a difference from the 4.08's?

You lose top speed from a gearing perspective, you dont lose power up top....

what's the top speed difference from the 2 sets? I wouldn't imagine it would be much? Isn't the car aero limited stock at 160 now anyway?

3.69 ring & pinion = stock 6MT

4.08 ring & pinion = 10% shorter than stock

3.9 ring & pinion = 5% shorter than stock

Not only that, but IIRC the 3.9 is an aftermarket and more expensive. IMO 5% isn't enough of a change to justify the work, not to mention it costs more. Just go with the 4.08's and call it a day.

I so regret not doing this when I installed my LSD.

anyone else want to chime in on if they are happy with their set-up if they have these gears?

isnt the RC 370z running 3.8/3.9 gears? im too lazy this monday morning to find the number.

The theoretical top speed for the 6MT 370 is 202 mph @ 7500 RPM. (275/35/19 tire, 3.692 final gear ratio, .794 6th gear ratio).

In stock form with the limiter removed the car is drag limited to around 165-168 mph in perfect conditions.

In 6th gear at 165 mph, you are at 6,100 rpm with the stock gearing. 5th at 7500 rpm will run up to 161 mph.

With a 4.08 final drive the theoretical top speed drops to 183 mph @ 7,500 rpm in 6th. At 165 mph in 6th gear you are at about 6750 rpm, though, so you are making more power in 6th at that speed than you were with the the factory gearing. Since you are making more power you will be able to overcome a little bit more drag, probably pushing your drag limited top speed up just a bit over stock gears.

The real gains with the 4.08 will be at normal speeds, though, since we aren't just out to do top speed runs. You'll get into the power band quicker.

Awesome, thanks for the info. When I said about losing top end I was more talking about the pull from 80-120ish... not so much the total top speed... i'm never going to be anywhere I could take the car to it's top speed limit.

4.08's are a worthwhile mod, although a bit expensive and difficult to install. yes you lose a bit of mph per gear but you can gain some back with a tune (raise the rev limiter).

he has stock gears.

How much do the 4.08 gears cost with installation? How much will my 1/4 mile improve by? How much MPG will I be losing if I'm cruising on the highway?

Assuming you have the grip to take advantage of the gearing, the 4.08s are probably good for a couple to a few tenths off your 1/4 mile time.

Highway mileage...

With the stock gear ratio, you are at 2,600 RPM in 6th gear at 70 MPH.

With the 4.08 ratio you are a hair under 2,900 RPM in 6th gear at 70 MPH.

You'll probably lose at least a couple mpg highway.

City mileage shouldn't change much at all.

at what point does gearing start to take away from torque and horsepower in the high revs? is it possible to overgear a car?

Yes, it's very possible to over gear the car!

Is there a magic number? Not really. But in general a 4.08 gear would be great for an N/A car if you don't mind the extra revs on the highway.

For a FI car it's probably best to stick with the stock gearing as it will surely overpower the low 4.08 ratio on street tires.

The price depends if you end up buying all the bearings & seals etc with the gears. I've seen the gears go for $500 alone, and I've seen full install kits go for up to $900. Installation would probably be a few hundred bucks.

1/4 mile time improvement? After 2 nights at the track w/ the 4.08's, I haven't been able to beat my previous best of 12.89 @ 107 yet. Still figuring out how to launch the car with the gears, tires and alignment.

MPG loss on the highway? I went on a roadtrip as soon as I got the gears done. I averaged 22.9 MPG over the 400 miles, mostly 70mph driving. Usually I can average 24 to 25 MPG driving 70+ mph. So yes, you lose a few MPG's on the highway. How much exactly depends on your driving habits.