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If you are considering, or have purchased, an 8, you will likely have one of 3 opinions on gas mileage:
Gas mileage is an often hot topic, and one of the big negative points about the RX-8 from online reviewers. They are correct, but only to a point. The RX-8 does indeed guzzle gas very easily. But it can also sip it far more reasonably than most reviewers are willing to acknowledge.
The reality is, a healthy RX-8 will get 17-18mpg city, 22+mpg highway. But 'healthy' means completely healthy: healthy coils, wires, plugs, O2 sensors, carbon free engine, good compression, healthy cat (or no cat), good transmission/diff fluid, no vacuum leaks, clean MAF, clean ESS. Let even one of these slide, and your mileage WILL drop, and it will drop fast. If you believe that RX-8s simply can not get this mileage because you have never seen it, then there is only 1 person to blame, and that is YOU. If you don't get this mileage, then there is something wrong with one of the above points. Or with your foot. Your right foot can make the mileage drop quite a bit as well. And I don't mean granny driving as the answer to good gas mileage. Smoothness is more important than speed. Smooth and fast is better mileage than unstable and slow.
Most official reviews of the RX-8 are done with the throttle used quite a bit though. Power costs gas, and no, the RX-8's engine doesn't extract as much energy from each fuel charge as piston engines. So if you are spending a lot of time at full throttle, enjoying the engine as it was mant to be enjoyed, you will get as low as 9mpg. Magazine or online reviewers often ignore this perspective. They spend an hour or so enjoying the engine and pushing it hard, and then complain about the poor mileage. Well yeah, duh.
Another common complaint about gas mileage uses the engine's peak power as a means of accusing the gas mileage. For example, I'm sure you hear frequently that that car gets 300hp and 30mpg, or whatever numbers. I understand where the motivation of that comment comes from, and it's simply that when they don't want to use power, they want good gas mileage. However, these two factors are not, and will never be explicitly connected. Cars that get good gas mileage get it in different ways. A small 1.5L Mazda2 gets 36mpg because it is optimized for mileage and low velocity pumping efficiency. A large engine producing 40hp at cruise will have greater pumping losses than a small engine producing 40hp at cruise. "But what about the Corvette! It has a huge engine and gets better than 30mpg highway!. The Corvette get's it's efficiency in another manner, torque. In 6th gear on the highway, the Corvette is basically idling, as it's engine produces enough torqu to easily provide the power needed to keep it at speed. There are very few cars that can do this, and it requires a number of factors to be successful. On both of these points, the RX-8 is already behind the curve. It doesn't generate any torque to speak of at all, so it can never act like a Corvette's engine, and it flows a LOT of air, which increases pumping losses when only a fraction of it's peak airflow is actually needed.
People also tend to point at it being a 1.3L engine, and assume that a 240/238/232 hp rotary "2-stroke" engine should be as efficient as a 1.3L 4-stroke piston engine (which is probably in the 70-90hp range). This simply is NOT the case. If you took a 1.3L piston engine and extracted 230+hp from it naturally aspirated, it too would have poor gas mileage and reliability.
So if 20+ highway mpg isn't sufficient for you, then perhaps you need to look at other cars. If you can't get that mileage, start poking around for what is wrong.
For reference, here is 8,330 miles of mileage data from my Sevenstock trip around the country in September 2010. The lowest tank was 13.99mpg, and that was in non-stop tight curves on the Pacific Coast Highway where I was having fun blasting along. Note the 4 tanks over 23.0mpg, 11 tanks over 20.0mpg. Also note how the highway mileage drops as speeds go up. Once you cross the ~70mph range, the wind resistance does start stacking against you more and more. It's easily a several MPG drop going from a cruise of 70 to a cruise of 80+. And those were probably closer to 85-90. The 18mpg in northern NV wasn't really highway, but it was alot of really straight empty terrain, and ... a lot faster.