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.

I was wondering when zatzuma would chime in on this thread. He tried the 4.08's. His results were: first gear was useless. Second was better, but pretty hard from a stop to start in 2nd. He changed it back to stock after a short time.

Most of the people interested in 4.08s are N/A guys worried about 1/4 mile times, though, so you'd imagine they'd be running a tire that could put the power down.

I did a 3.42 to 4.11 on my last car, Camaro SS, and the difference in performance was astounding. You feel it most in the mid range, the engine has a much better mechanical advantage over the wheels. This is a significant loss of fuel economy when driving on highway. If you drive a lot off highway it's not so bad because you can simply shift at lower rpm to compensate, which the higher gear is happy to do.

Oh, and in my Camaro (post above), with the 4.11s the gearing was similar to the stock MT6 Z between rpm and speed. So in the Camaro 1st gear was still usable for normal driving, but I would never use more than half throttle, just too violent and too easy to spin. In the Z, I agree that 1st gear will be useless with the 4.08s, you'll never need it.

Not quite. I changed it over to the 3.9 gearset (5% increase)

Like it much better for all around driving. 1'st gear is out of RPM in 2 sec or less at full throttle. The 4.08's ran out of RPM even faster! It's no fun bouncing off the rev limiter that quickly.

Now that I've used the 3.9 gears for awhile, it seems normal for me to shift to second almost immediately after accelerating from a stop.

The rev matching (SRM) is a lot closer with the 3.9's vs the 4.08's.

In retrospect, I would have left the 3.7 gearset in with the supercharger as I'm always loosing traction in the first two gears at full throttle.

But in 3'rd and 4'th gears and the SC the acceleration is fantastic ...... right wheee!? :stirthepot:

Trying to decide which final drive would be best. I have a bone stock 09 370 nismo, that i plan on boosting with the gtm TT kit. This kit alone will give me approx 500whp. However, i want to do gears first. Any thoughts on this delema?

FWIW, I'd have the shim kit on hand before starting the swap. It's faily inexpensive, but absolutely vital if you need it. If you don't need or use it, you take it back. If you need it, you could possible find yourself with downtime tracking the shim kit down.

Will a 7AT loose cruise control with a manual differential running 3.9 gears?

This is what I was thinking of doing with mine... From the 7AT ratio of 3.3 I believe? :ugh2: To just going for a Manual pumpkin which is still better than ours, instead of a full 4.08 swap... would this be worthwhile? Or a waste of time... and how does it affect drive-ability?

- Cruise Control
- Speedometer accuracy
- Tire Sizing to Compensate??

Will UpRev take care of recalibration of the speedometer? Is there an option to input your final gear ratio in to make that correction? I was planning on scheduling an UpRev tune as soon as I install my exhaust the end of the week, but will change out the rear diff first if the UpRev will make the speedo correction too.:confused:

Quote:
Originally Posted by daemon1957
Will a 7AT loose cruise control with a manual differential running 3.9 gears?

This is what I was thinking of doing with mine... From the 7AT ratio of 3.3 I believe? To just going for a Manual pumpkin which is still better than ours, instead of a full 4.08 swap... would this be worthwhile? Or a waste of time... and how does it affect drive-ability?

- Cruise Control
- Speedometer accuracy
- Tire Sizing to Compensate??Any help out there? Please:hello:

In another post I asked about CC as I do a lot of freeway driving. The lost in mpg doesn't really matter to me. I've been told that yes for sure you lose CC as the wheel/speed sensors are affected. But then I was told by another member that he's works on and off. Are there no tunes that can fix this? I've talked with a few local shops and no one has an answer.

I wanna do 4.08's on mine but I was told that by changing tires is like changing the gearing ratio? I had never heard anything about that. Can someone confirm how accurate this will be. Im running drag radials 285/40/R18. Wel Im one of those guys that is worried about 1/4 mile times. But I dont have money for FI. I wanna get my Z into the high 12's all day. Thanks

That drag radial is slightly taller than a stock sport package tire.

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
275/35-19 3.8in 13.3in 26.6in 83.5in 759 0.0%
285/40-18 4.5in 13.5in 27.0in 84.7in 748 1.5%

Thank you, I gave u some rep for that tranceformer. So ive increased my gearing by 1.5% or the tire? That is very minimal. To where the driver cant tell the difference. So if I install some 4.08 gears I should still benefit from ~10% acceleration increase correct? I was told it wouldnt make much of a difference once the tires were in place.

Your actual speed will be 1.5% faster than what the speedometer reads.

4.08 gears = ~10% more aggressive than 3.7 gears, but not necessarily 10% faster acceleration.

Stop speculating and just get the gears installed. ;)

I bought the 4.08 gears but the don't fit the diff hemisphere. Anyone know why

I swapped my gears for 4.08 when I changed my LSD and have no regrets . Car launches way better and goes through gears like lightning . In for 4 years with zero problems to date .

Also, not sure why everyone says first gear is useless ??? U burn through it a lot quicker but my oh my it's fun ..

I'm guessing the 4.08 makes the gears longer. There's no way anyone uses 1st gear at wot. Too short on stock gears, almost no traction at wot.

Is anyone using the 4.08 0r 3.09 gear with a supercharged ride.
I am looking at this mod and would like some advise - to do or not to do , 4.08 or 3.90 with supercharger

Is this mod affecting idle rpm's?

has anyone found a solution to "losing" revmatch when going with 4.08? since the 15 nismo has 3.98 or whatever, does this mean nissan has the ability to "tune" the revmatch system? wonder if they would be willing to do that for you.

no, its the final drive ratio. Only effects the mechanical advantage of power through the driveline when the car is in gear. engine/flywheel/clutch or torque converter if your auto/transmission/drive shaft, rear diff "where FD is located"

The final drive determines the ratio of how many revolutions the driveshaft makes to turn the rear differential or tires one turn, or 1 foot radius to be more precise. if its a 4.08 to 1 the drive gear rotates 4.08 times and the driven gear rotates once.

Rev match will not be lost. I believe it is based on engine RPM, trans input & output speeds.