If you think 24 mpg out of a 2.0 liter engine is bad:
http://www.fuelly.com/car/mazda/rx-8...arakawa/392021
There is a lot more to consider than the amount it costs to fill the tank and the distance driven between fuel ups to determine how efficiently an engine makes power. There are also some flaws in using only that, unless the driving environment and cost of gas per liter were identical in both situations. That's not even getting into power output or fuel delivery, as well as countless other variables I'm too lazy to list.
If you're not happy with the current efficiency in your FRS, flash it back to stock and be sure to keep the Primacy (or comparable low rolling resistance) tires. If you're still not getting comparable efficiency to others with a stock car, there may be an issue with either the car or driving technique. If you're not happy with the average fuel efficiency others are getting for the engine displacement and power, research your next vehicle more carefully prior to purchase.