Possible to regear RX-8?
 

I know I am a newbie to RX8's and this forum, but I have a few findings I want to share.

I am using a Scanguage to monitor my fuel mileage, and I do have it calibrated correctly.

Driving 70mph in 6th gear I get about 21-22 mpg on average. Driving 63 mph in 6th I average 24-25 mpg. And the other day I went on a slow drive and found that in 6th gear on level ground at 45mph in 6th gear I could average close to 30mpg out of the rotary.

Now, I have always heard that rotarys need to be wound up all the time to run properly. But, I'm really starting to not believe that. The car pulled it's weight just fine without over loading the engine. Anytime I would try to run a lower gear / higher rpm, the fuel mileage would drop in a hurry.

So, is it possible to regear the RX-8? I really believe if it were geared to run lower RPM's it would get much better fuel mileage at highway speeds. If it will pull itself in 6th gear at 45mph then why couldn't it run about 1,000 rpm less at 70 mph? Has anyone ever tried it? I wouldn't even begin to know where to start looking for parts to do it.

I know some people will say that the car is meant for performance not fuel mileage, but it would be worth it to me if it would be economical in the long run.

Very first part to consider is replacing the rear differential on your manual car with one from an automatic. If you can find one with limited slip, then you're doing really well.

BC.

You're posting in the Series II forum. You have a 2009 or later? If so, these come with a 4.77 final drive ratio. The earlier manual transmission cars come with a 4.44 ratio. Swapping in one of these will drop your highway cruise revs by about 300 rpm. To get a 1000 rpm drop would require something like a 3.1 final drive, which doesn't exist for this car as far as I know.

I use a 3.9-1 rear gear set out of an auto FD RX7 in my 6speed RX8 and that drops the revs by around 5-600rpm in 6th gear. Mine is a series 1 though, so if yours is a series 2, this may be a little more or less depending on the gearbox ratio's.
Same rear gear set as the auto RX8.

Rotaman

This is very interesting. Does the speedometer read correctly?

And to add:
My RX-8 is a 2009 Sport, Manual Transmission

Read more.

My speedo does not read quite right, but that is not to do with diff ratio, but because I run 19in wheels with low profile tyres. The speedo is read through the wheel speed sensors, so tyre height is the important thing to get right.
My tyre height is smaller then factory, so my speedo reads a little faster then it actually is.
If you are using your car for mainly long distance driving, then running at the lower revs is fine, you shouldn't have any problems.
If you use your car for a lot of around town or short distance, then you may end up fouling plugs and engine carbon buildup if you are only going to run at low revs.
The other way to help stop fouling plugs and building up carbon in the engine is to use a warmer set of plugs, but remember to change them back to the original set if you are going long distance.

Rotaman

You won't get that much better mileage IMO. The friction/drag difference between 45 and 70 is significant. It will take so much power to overcome it. You're not accounting for load. The slightest upgrade, wind resistance, etc. will cause large load variations because at such low rpm the ability to make torque is significantly reduced.

Gearing doesn't affect the speedo accuracy, tire diameter does

.

The biggest thing I ever did to get better highway gas mileage was to use the cruise control like it's my religion. Those little variations in speed that happen when driving with your foot on the gas are SO easy to overlook in this car, but they waste a ton of gas on long trips.

Also, the A/C turns on automatically when you use the half-windshield/half-footwell vent setting, so make sure to avoid that unless you need to de-fog the windows.

For a small difference you could try switching to some larger diameter tires. I'm considering 225/50/18 which would drop 3500 rpm down to 3383 rpm.

Aero drag is probably a more significant factor jumping from 45 to 70 although I would expect the engine to be more efficient when working at higher loads and so efficiency would increase with lower rpm.

The NC MX-5 and the S2000 have a 4.10 FD, which IIRC both would work. You might even get someone to pay you to swap gears. Another option is to trade the 19" rims and tires for a set of LRR summer tires(they do exist) and smaller rims. But that's only worth it if you're going to replace the tires anyway and can get your money back on the rims.

Worst case, you could drive 45MPH everywhere. :rofl:

False, NC, s2k and rx8 gears are not interchangeable.

I wouldn't run lower than 2.5k rpm anywhere anyway. At low rpm you have all kinds of weird seal chatter and a bunch of other problems.
Why don't you just tune to stay in closed loop a bit longer and then try to stay in that rpm range? That would save you some fuel without touching your car's performance.
The rx8 is already underpowered and heavy, add a taller final gear and you have a sluggish car.

Do you know what is?

Some previous rx7 final gears.

faster = more gas burnt not because of RPM engine is turning at cruising speed.
re-gearing will not give you better gas millage and will possibly make 6th gear too tall for cruising speed.

the FD and Rx8 Share ring and pinion sets, some share part numbers. the P type diff is an 8" ring gear.

the A/T FD's have a 3.9, US and 16" JDM FD's are 4.1, and the 17" wheel FD's are 4.3.

the 2004-2008 Rx8 is 4.3 or 4.44

the 2009+ is 4.3 with A/T or 4.77 with MT

the S2000 uses the Mazda M type, which is a 7" ring gear, its the same as the 1970-1992 non turbo rotaries, and 94-2005 miata. might be why its a little fragile in the Honda, they are putting nearly twice the power thru it than Mazda has ever done.

i'm not sure what the 2006 miata uses, its different part numbers than the Rx8 and early miata stuff, so it could be something new, or rebranded old

It's a well known fact that taller gearing improves fuel economy. If you ever look at AT gearing it's much taller. You'll see that they have to be going much faster to even use their top gear. Typical is anywhere around 45MPH for AT but in MT you might only need to be going 35MPH to maintain speed in OD. That is due to taller gearing. MT are all much shorter than AT nowadays, so even though a car could pull taller gearing they are all given shorter gearing for better acceleration.

The AT in the S2 has a top gear of 2.5(OD of.582x4.3 FD) vs the 6spd with a 3.76 (OD of .787 x 4.777 FD).

Very interesting. I could think of a few applications for this; turbo cars or engine swaps. Not very practical for a N/A 6port though. The cost is just a bit too high to make this worth it. Even if you get lucky with a DIY $600 diff and save 10% on gas, that's two maybe three years depending on how much you drive. Probably the last thing I would do personally.

Rotaman, I plan to use the 3.90 gears from the Auto FD RX7, but would like know your experience with the set up. How is the around town driving and is the highway travel at 75-80 mph(120-128 KMH) getting you better fuel economy?

dwro, I have been using the 3.90 gear set for a few years now and have found it to be good for what I use the car for, which is mainly long distance driving.
Don't forget my car is turbo'd so with the taller gearing I get probably slightly better fuel economy then when my car was standard. But I contribute that to two things, one the taller gearing, so its not revving so hard in top gear and the fact that my car is turbo'd I don't have to work the car so hard to achieve the same thing. Put it this way would I want to go back to the factory gearing, NO.
I can only compare this to the other rotaries that I've owned and when I first drove the RX8, I kept wanting to change up to another gear on the open road as it always felt like it was revving to high for me.
Hope this was helpful.

Rotaman