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ASSUMINGthat your engine, ignition, BOTH O2 sensors, MAF, e-shaft sensor, and fluids are all healthy AND your catalytic converter is either healthy or removed AND you do not have any vacuum leaks, then these are some options for improving mileage:
This is completely a scam.Yes, in theory it is possible that a smoothing and 'spinning' of the intake air flow can feed more air to the engine than a stock intake, any improvement requires that the stock intake be designed incredibly inefficiently, AND the placement of the Typhoon is such that there are NO bends, kinks, splits, etc... between the Typhoon and the point that the air is actually getting into the engine. In today's world of mileage at the core of marketing, there are no OEM intakes which are inefficient enough and simple enough to meet these requirements.
There are potential benefits here. Functionally, an engine is at it's efficiency peak when it is also at it's torque peak. So moving the torque peak to where you drive at will have some small minor mileage gains. However, this isn't really doable for the RX-8, as our torque peak is largely based on the design of our intake. You will have to redesign the intake to move the curve significantly. A tune specific to your engine and ECU will produce some gains, but they will likely be 1mpg or less.
The only other tuning impact is with the still-being-explored method of RX8Club.com: Lean burn with negative split timing. It has been shown to momentarily increase mileage by around 50%, however there are significant engine damage dangers in doing so, and significant power loss while under this condition. The ECU also refuses to allow this condition to persist, and will bring the engine back into expected norms before long. The tolerance of the rotary to this is greater than expected however, so it is something that Mazda may leverage in future rotaries if they can solve or address the dangers associated.
There are only 2 practical gearing changes available, swapping between the series 1 and series 2 transmissions, and swapping between their rear gears. With the exception of some wiring, the transmissions are compatible. The rear differentials are compatible. The series 2 RX-8 got a taller gear to assist with acceleration, however the transmission gearing was changed so that this wouldn't impact the highway mileage, only lower gear acceleration. Between the 4 possibilities though, you can find several gearing options that may work better with the rest of your mods to assist mileage. Several RX8Club.com members with forced induction are approaching 30mpg with light highway cruising and gearing changes. If you stay naturally aspirated, the gearing is unlikely to provide significant benefits.
I have personally seen 24mpg in my RX-8 in normal highway driving without focusing on mileage, and everything was healthy. When hypermilling, I was able to repeat 27mpg a few times. Smooth throttle usage is the most important trait, and to hit that it was basically "no throttle usage". Steady state speed for hours on end. Frankly, it's rather boring. My mileage drops as the speeds go up, and as I get frisky and wanting fun. This is true for you as well. If you are serious about saving gas on the highway, set cruise control to 65mph and leave it there, dodge all the traffic pounding down your ass in the rear view mirror, and assuming everything about your car is healthy and the road is flat, you will be able to hit 23-24mpg. Bump that to 70mph, and it's probably going to drop to 22. 75mph gets about 20, and 80mph drops it to 18-19. Air resistance is unavoidable. But it is also something you can mitigate a little bit. AC on and windows up is better mileage over ~65mph than AC off and windows down. Under ~65mph, windows down and AC off produces better mileage than the reverse.
The big killer of any highway mileage is aerodynamics. You can improve your car's co-efficient of drag in a few ways that are simple, like leaving the moonroof and windows closed. The OEM spoiler reduces the coefficient of drag by a point or two (but not "wings"). Beyond that, you have to get more drastic if you really want to make improvements here: