I have a 2004 Coupe and was not thrilled with the 251HP result after Uprev, and thinking my rear tires may be one of my problems. My rear tires are 275/30/20, and I'm guessing the larger diameter is really hurting me. I have MMR GT1's and love the rims and they go perfect with the car, so I'm trying to find an alterative to replacing them. With that said.. Which Final Drive will counteract my larger diameter rear tire? would I have to step up 1 ratio or 2? Thanks

 

https://www.z1motorsports.com/upload...GearRatios.pdf

 

I have 285/30R20 rear tires and they are a perfect match for the diameter of my stock 19" tires.

Few things going against you right now. 1) larger wheels/tires are heavier and rob horsepower trying to spin "unsprung" weight. 2) age. Car is 14 years old, engines wear out and lose some HP over time.

The car can feel a little faster off the line with shorter gears, like 3.69 from a G37 6MT. Or visit Z1 for other options. I think the most common are 3.69, 3.9 or 4.08

 

Auto or manual? You probably have a 3.3 ratio. A 3.7 would be a noticeable change. If auto, I wouldn't go any higher than that due to high RPM's at highway speeds.

 

so 20" rim with 30 profile tire is the same diameter as the stock setup? I thought I was maybe 1-2 imches more and that was my problem..

I have a manual, and really would like to go the highest while being reasonable, especially since Z1 lets you pick any gear ratio and its <$600.. If I remember right, when I first got this car and was looking into final drive gear, some were pretty expensive. What would be a good FD for a manual?

Thanks for the replies guys

 

A 285/30R20 is the same diameter. 275/30R20 is a little smaller in diameter. You'd have to compare it to your stock tire size.


As far as what's best, it depends on what your intended purpose is. Going with shorter gears affects top speed and cruising RPMs. If you have a manual trans, your ratio is 3.5. Use that as a starting point in online calculators.
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OK OP here's my input. 251HP with minimal mods is excellent (depending on what type of dyno you were on) so there's no problem with your car or tires. As others have mentioned differentials don't create power they just take it to move the wheels. Higher ratios might feel faster when you get on it but you lose that momentum as RPM's increase. You also cruise at higher RPMs at the same MPH, if you do a decent amount of highway miles then your gas mileage will suffer as the engine's working harder that it did before to maintain speeds. I swapped a 3.5 into my 5AT 05 sedan a long time ago and it made sprinting around town more fun. But when I got on the highway and was running 3200 RPM's at 80 MPH (legal speed on a lot of Texas highways) my gas mileage dropped 3-4 MPG's.

For comparison my 06 M6 GTO lazily cruises along at 2000 RPM's running 80. God bless the LS2/Tremec T56 6 speed combination.

 

Was just expecting more seeing as how a stock G35 6mt is around 230hp to the wheels, and I have tune, nwp throttle body, z tube, hfc+catback, and plenum spacer and only 20 over that. And I get it, could be the dyno machine some read higher/lower, and the car isnt new, but still.
Theres a ton of threads here on final drive and after reading I might do it anyway. reading up on all the pros and cons, and I wouldnt mind sacrificing a few mpg for better 0-60 which it looks like a FD will exactly do, especially because sometimes on the freeway I dont even get into 6th gear. having $1200+ in tickets and license suspended 3 times in the past 10 years while I had my old car, I dont speed on the freeway when highway patrol might see you from 1/2 mile back or on a merge, just the occasional 1-2-sometimes 3rd gear pull and even thats not often, but nice to have a little more there. just looking into which one, and multiple posts saying its one of the best mods for the $$ in a 6mt.. damn, and I thought after the tune I was done...

 

Honestly I'd look into freeing rotating mass elsewhere like an aluminum driveshaft before I went swapping out gears. About the same cost and return in power.

 

huh, when I saw Aluminum/CF driveshaft's in the past I knew it was reducing weight, but never clicked that it would free up rotating mass. Just figured it was another mod for weight reduction, will definitley look into it. Thanks

 

Yeah flywheel, propeller shaft, and brake rotors make a HUGE difference when you combine all 3.

 

Subscribed.

 

I'd go (3.9 ring & pinion) final drive gears. A lot of ppl say it is too much but if you are not forced induction there is no better performance upgrade for the money. If you really want to maximize gains, I'd get a set of used lightweight 17" wheels & some sticky 245/40R17 tires. That creates an alternative set of bolt-on "running shoes" you can easily put on at home when you really want to go out stretch out it's legs and remove when finished. With the lightweight rotors, prop. shaft, and driveshaft, also, the new rollers will pull much harder. Especially an eighth of a mile or stoplight to stoplight, though this is metaphorical and I would never advocate racing on a public street. Too much can go wrong and it's illegal. Anyway, the less rolling resistance of the 17" (hell, even 16" would cut some throats also, metaphorically) wheels on occasionally spirited evenings of driving will be your best bet and your total final drive ratio would go up beyond 4.10. Only reason I say anything is because I used to ride stock 19"ers (with a 350z manual 3.5 final drive carrier w/ VLSD) in my 5AT and when I made the same changes I listed in this reply I got better gains than any other performance mods I had previously tried. Now I am going to 3.9 gears on my 17"x8"s and after I felt the amazing benefits of the 17" diameter with great tires, I wont go back to 19"s unless I do a show or something.

A lot of great info for you in this thread, hope it helped ya out, bro'

 

Heavy 20" wheels aren't a good idea if you're after performance. Just going from the original 16" wheels on my Accord to the 16" RSX wheels, I saved 9 lb per wheel in weight and the difference was noticeable off the line, in throttle response and braking. Looking for some lighter that stock 17 x 8 wheels for my G35 so I can save some weight and put more tire in back.

Believe my 6 speed car comes with 3.54 gears? We get my wife a G37 we won't need my car for road trips anymore so I'm considering 3.90-4.10 range for mine.