What would it take to get an RX8 to get 30 mpg on the highway?

I’ve averaged 21.4 mpg in my RX-8 over its 224,000 mile history but I carpool with someone so I’m actually getting a hybrid matching 42.8

I’m turning a ridiculous 4,030 rpm’s at an indicated 80 mph in 6th gear. Mazda bakes in a lot of speedo error so I’m really only getting something like 74 mph on those 4,030 rpm’s.

It seems to me that just swapping in a taller differential would be an easy and effective way to significantly improve mileage.

I climb a max freeway inclination on my daily commute and even if traffic slows me down to the high forties, I have no trouble pulling the car right back up to speed while leaving it in 6th gear when an opening presents itself.

Basically, six speeds but none tall enough.

singleslammer wrote: In reply to oldeskewltoy: Does the 8 and the IS200 have the same transmission? I am guessing yes. The 4age is an interesting option...

They both have a version of the AZ6 from Aisin Industries. The bolt patterns are a bit different... but LEEN has taken that into consideration and has provided both bolt patterns into the adapter....

Also... we have a prospective client who we may change the bolt pattern to accommodate the 6 speed behind other engines.... right now the face of the adapter is drilled only for the T50 bell housing

Another engine option for you without changing the gearbox....

Nissan's SR20DE... or SR20DET. The DE can be made to pull great economy, and as a 2 liter can provide 200hp, the SR20DET can provide you the same kind of power an LS swap will provide....

SR20DET at LSx power is going to be (probably) more dough and smaller powerband (I have an SR20DET in my 240SX so I'm not bashing it). It is a light engine, though.

Knurled wrote:

Snrub wrote: You might not be able to get 30mpg on the highway, but the savings between say 25mpg and 30mpg is fairly small. To maximize fuel economy drive a bit slower. I find fuel economy is a fair bit better around 60mpg than say 70mpg.

When the RX-8 was new, a forum member tried driving at different speeds on cruise control. 60, 70, and 80mph were all within a half MPG of each other. (All around 23-24mpg) The difference between 25 and 30mpg is huge. Another way to see it is, over 150mi, 30mpg is one gallon ahead of 25mpg. Over 3000mi, it is twenty gallons ahead...

Another idea would be to try running lower octane fuel. It might not help with economy, but it can represent a ~10% savings on fuel.

In all cases but two where I've experimented, higher octane fuel netted better fuel economy because the computer was able to take better advantage of it. On one where it didn't, the engine was a scrody no-compression pre-knock sensing POS. The other was my RX-7 that runs better the LESS timing you give it, so it would probably get best fuel economy at ATDC timing and running on kerosene and lacquer thinner

The originator of the thread mentioned this stuff isn't the point of the thread, but I'll still address the comments. I'm not sure who the forum member is that you're referencing regarding fuel economy vs. speed, but you're basically arguing I'm wrong because someone else found contrary evidence. That's fine, believe who you will. I'm not claiming I've ever achieved a remarkable lifetime average or even one trip fuel economy, I'm merely pointing out what my logs suggest has produced better fuel economy. I've experienced reduced fuel costs on similar long highway drives when running 87 octane. That said I don't run 87 often. If my memory serves me correctly, there was a GRM article about the RX-8 a few issues ago where one of the major performance shops (Dave Lemon/Mazdatrix?) suggested they run 87 all the time. Regarding the fuel costs, look at it this way, 30 vs 25mpg is 20%. If someone spends $3k/year on gas, 20% is $600 and if half of it is highway, that's $300 a year (use whatever ratio you like, increase overall cost, etc.). It takes a long time to make up say $10k.

NONACK wrote: Doesn't Swank Force One (the car) get surprisingly good MPG? What would it take to put an F2T in one of these? Swank Force One (the person)?

I think you could probably get an F2T in one of these without the help of Swank Force One (the person).

However, this is going to require re-wiring of the entire car. From scratch. Custom gauges, the whole 9 yards. At least... as far as i know. Or at least, that's what i'd do.

I'd be up to the challenge.

But yes, Swank Force One (the car) gets ridiculous MPG. Combined hasn't been below 30mpg in about 4 years. Typically with my normal 60/40 highway/city split it sits around 33-34mpg. Best highway trip to date was on the way down to GRM $2012, where it got 41mpg.

The car weighs 200-300lbs less than it did back then, and no longer has a blown head gasket and blown turbo. I'm expecting an improvement from that point when it runs again between the fixes, weight reduction, and further tuning. I would not be surprised to see 50mpg out of the car with some light aero work and a 3.85 final drive swap.

Low compression, high torque is the key. If i had an actual good way to put the car in lean cruise, this would be cake. (Turbo Miata runs in the high 15s/low 16s AFRs at cruise and has done 35mpg highway, despite short gearing.)

I don't know how similar the RX8 6spd is to the Miata 6spd. I do know that the Miata 6spd actually bolts up to the F2T after about 5 minutes spent drilling two bosses out on the bellhousing. From there it's light modification to the clutch fork arm thingymabobber.

You'd have to mess with motor mounts, then getting a long rear end under the thing, since now you have a motor with a 6000rpm redline. And of course... wiring.

If you want a stronger transmission, you need a B2600i trans in its entirety, or a B2600i bellhousing on an FD transmission. At that point, you'd have to worry about tranmission mount.

Swank Force One wrote: I don't know how similar the RX8 6spd is to the Miata 6spd.

The bell end, and the tail housing end are different.... the center section of the case, and 5 of the 6 gears are the same........

Snrub wrote: but you're basically arguing I'm wrong because someone else found contrary evidence. That's fine, believe who you will.

It certainly would not be the first time somebody has found a weird kneepoint in rotary fuel economy. Even with the benefits of side exhaust ports, they still lose a lot of heat to the cooling system so they are remarkably inefficient at low load.

If my memory serves me correctly, there was a GRM article about the RX-8 a few issues ago where one of the major performance shops (Dave Lemon/Mazdatrix?) suggested they run 87 all the time.

That... all depends. What octane is best for N/A rotary is something that gets argued a lot. Some like 87 some like 92 some like race gas. A lot of it is dependent on the actual car. I see minor power gains with 92 but my ports don't like having very much ignition lead at cruise, and still using a distributor makes it make mroe sense to just dial the timing back and run 87 as the best compromise.

A dealer guy I communicate with says it is normal for RX-8s to have low RPM detonation, which leads me to believe that there are definite benefits to higher octane since Mazda saw fit to give them that much timing.

Regarding the fuel costs, look at it this way, 30 vs 25mpg is 20%. If someone spends $3k/year on gas, 20% is $600 and if half of it is highway, that's $300 a year (use whatever ratio you like, increase overall cost, etc.). It takes a long time to make up say $10k.

You're looking at it from wrong perspective. Put a piston engine in car, car becomes incredibly boring to drive, miles driven drops to 10% of what it was with rotary. Massive fuel savings!

oldeskewltoy wrote:

Swank Force One wrote: I don't know how similar the RX8 6spd is to the Miata 6spd.

The bell end, and the tail housing end are different.... the center section of the case, and 5 of the 6 gears are the same........

Are bells swappable? Or is one/both 1 piece cases? If the input shaft length is the same and you can throw a Miata bell on it, that would make the swap REALLY easy, mechanically.

Swank Force One wrote:

oldeskewltoy wrote:

Swank Force One wrote: I don't know how similar the RX8 6spd is to the Miata 6spd.

The bell end, and the tail housing end are different.... the center section of the case, and 5 of the 6 gears are the same........

Are bells swappable? Or is one/both 1 piece cases? If the input shaft length is the same and you can throw a Miata bell on it, that would make the swap REALLY easy, mechanically.

photo above shows Nissan bell on RX8 transmission... so fitting a Miata bell will not be an issue

oldeskewltoy wrote:

Swank Force One wrote:

oldeskewltoy wrote:

Swank Force One wrote: I don't know how similar the RX8 6spd is to the Miata 6spd.

The bell end, and the tail housing end are different.... the center section of the case, and 5 of the 6 gears are the same........

Are bells swappable? Or is one/both 1 piece cases? If the input shaft length is the same and you can throw a Miata bell on it, that would make the swap REALLY easy, mechanically.

photo above shows Nissan bell on RX8 transmission... so fitting a Miata bell will not be an issue

Ah ok, my bad. Pics blocked at work.