Rear End Ratio & MPG
 

So in 09 (?) they lowered the gear ratio in the RX-8's rear end, and as a result, the car accelerates faster. I owned an 04 and an 07, and today I test drove an 09. The 09 definitely picks up quicker. But is the lower gear ratio also the reason it went from 18/24 MPG to 16/22 MPG ? Could one get an aftermarket rear-end at a higher gear ratio, something higher than even the old default, to get slightly slower acceleration, but better gas milage? I don't buy these cars to race them, I buy them to be daily drivers that are fun to drive. If I could eek a few more MPG high out of this car, that would be great. And I'll just put the original rear end back in when I go to the track, which will be absolutely never.

So you want to spend a couple hundred dollars to buy another rear end, swap it in, all to get one or 2 mpg more??? I see it taking a long time to get a return out of that. Not worth it IMO. Plus these cars have no torque so I would think it would cause more harm then good.

Although changing the differential final ratio can impact mileage and acceleration. If you check the following site, EPA changed their method for mileage testing in 2008. As a result, the mileage estimates are supposed to be more "realistic". The mileage estimates for the 8 are shown here:

http://www.fueleconomy.gov/feg/calcu...umn=1&id=19485

It appears that the difference is due to the new testing procedure rather than gear ratio (which would be a smaller impact to mileage).

If you are really serious about swapping the rear end gear, I am 100% sure you could find someone that auto-crosses with a series 1 that would love to swap diff's with you. Probably the cheapest option.

(I am assuming that they are directly swappable, though you might want to check first)

ok. afaik, you have several facts wrong.

1. the rear end ratio was increased. from 4.44 to 4.77. this will make you accelerate FASTER and was done to accommodate the lower gearing in the transmission.

2. the 09 accelerates slightly SLOWER 0-60.

that said

you can put a 3.9 rear end on the car, that will work. and you will get better gas mileage. probably in the range of 25 highway if you cruise at 70ish.

Has anyone done this or even higher ratio: high 2s or low 3s? I am aware of torque, mileage, 1st gear etc. I'm just looking for lower ratios overall, preferably by ring & pinion only... haven't found any sets yet, but I'm sure they are out there?

It wouldn't be a smart move, the gearing would suck.
If you need better mileage there are many diesel cars that do over 45mpg!

If your interested in trading rear ends for a S1 im willing and located in western Massachusetts

I still sell every available ratio including the 3.9. I can do ring and pinion or a complete unit.

Paul.

I changed the rear end in mine from the factory 4.44 to 3.9 and the difference on the open road is quite noticeable in 6th gear, where there is a drop of around 500-600rpm crusing at around 120kph ( 75mph ), which for me was great. I didn't like the fact that with the standard 4.44 ratio, I felt that it needed another gear so that it wasn't revving so high all the time.
I used a gear set out of the fd RX7.

Rotaman

Very interesting ... any impact on fuel consumption?

I think the fuel consumption is probably a little better, but I didn't change it for the fuel consumption, I did it so I could drop the revs in 6th gear when crusing long distance. I can usually get around 450ks per tank, which isn't too bad considering how bad NZ roads are. Also my car is turbo charged.

Rotaman

Ha yes that's awesome to know, turbo'd as well? Is the set-up I was eventually working towards... How do you like ur low band power?

I drive much hwy, have 148k miles on an '04. Getting even just 1/2mpg more than the 19.5/23 I'm getting now would be a major savings...

I drove the RX-8 in 2nd for a week so see how I would like the gearing. 1st Final gearing is 16.7 & 2nd is 10.1, I could certainly live with +13-ish. I think my goal would be to keep the secondaries closed at 75mph, but still researching that.... You're right in that I wouldn't want any lower than 3.10 in the diff for sure.

The 38-40mpg euro TT TDi is the only other car I'm remotely interested in, idk why ha.

Thanks rxomeo, but I'm looking for just the ring & pinion though for now.

Ok Paul, let's get in touch, thanks.

I find now that its turbo charged, that the low end torque makes a huge difference to how you drive it. You don't have to change down a gear to pass people, just push a little further on the gas pedal.
I think if you drive mainly highway miles, then the 3.9 gear set would probably suit you well.

Rotaman

Any contact for Paul after a diff set

has anyone else done what rotaman has done? i am curious about this being turbocharged, I find it annoying cruising at 4000rpms at 75mph

I'm pretty sure the 3.9 gear set is out of the fd RX7 auto rear end. I must admit I have been very happy with the change to a taller gear set.
I now cruise around 3000 rpm.
There are other ways to achieve similar results.
Brettus changed his 6speed to the 5speed gearbox and got a similar top end ratio.
The other way would be to change to the S2 RX8 6 speed gearbox, which I think would also give you a drop in top end revs at the same speed.
Hope this helps

Rotaman

4.444 gear's are your sell them ?
 

Hey Rotaman
are you selling them gear's 4.444 or know anyone with 4.777 gear's
thanks

Sorry don't have the 4.44 gear set any more, and unfortunately I don't know anyone selling a 4.77 gear set.

Rotaman

Suppose the opposite direction
 

Let's suppose that instead of a taller gear set, like the 3.9 from the FD, I want to go with a shorter gear set like the 5.12 Mazdaspeed. As near as I can tell, this would give me about a 15% increase in engine speed at any given speed in a given gear as well as 15% increase in torque at the rear wheels; but I'm hard pressed to predict how bad the fuel economy will be degraded. My car (2004 GT) has never been on a chassis dyno, so I don't know how the fuel consumption varies throughout the rev range.

It's not just a matter lowering rpm. An NA Renesis doesn't generate enough torque at the resulting rpm to make any advantage of it. No different than trying to run the current rear gear in 6th at low rpm. Try it and you'll see what I mean. You have to stand on the throttle to get it to barely accelerate. That's not how you conserve fuel. A turbo conversion is a different story because it makes the engine generate better torque down low (assuming it's done right).

Raising it with a 5.12 will result in buzzing around at too high an rpm all the time. Too high is as bad as too low.

and the OP's premise was flawed from the start. The 2009+ 6spd manual does not accelerate as well as the S1 6 spd manual all else being equal. He was probably comparing it to a worn/slow S1.


.

TeamRX8 is right. the 5.12 Mazdaspeed gear ratio is way too low. when I was building offroad trucks and Jeeps, I would swap in the 5.13 gears front and rear to offset huge mud tires like 38 to 40 in tires. for the RX8 though, I would say a 4.77 would be a good way to increase a little off the line acceleration without extra high rpms when cruising around.