how do u know which flash do u have on ur 8?
 

sorry i am new and just got my 8 and i guess the m or n flash will get better gas mileage. how can you tell what flash do u have on ur car? i just got my like 3 weeks ago but i am getting like 14.5 mpg so it's really bad.

There is no way for you to tell. But there are some clues. The build date of your car and was the fuel tank recall ( 8/2005) done. That recall ( minor issue ) included a flash (R-Flash), the most recent flash. You can ask the dealer, look for a blue ' 3305G' sticker under the hood or call Mazda USA to check if that recall was done. If your car was built after 7/2005 ( check sticker on drivers door jamb ) you have the most recent flash. How many miles does your car have and how long is your avg. trips? I get around 20 MPG doing mostly Hwy miles. If your driving is mostly around town you will get much less.

my car only have 750 mile but i am getting less then 15mpg and no way near 18

Your car is still very new and if you drove your car the way the EPA did LINK you would get 18 +.
This subject has been beat to death here since 2003 and the basic answer is the RX-8 sucks mileage wise. I get 20 MPG because 80% of my driving is in 6th gear. If your time in 6th is lower so will your milage be.

Yea I have mixed driving every day and get 14mpg. Woohoo!!

Just take the car to the dealer and ask them to check. I just walked in, asked them and the service manager brought the little box out to my car, checked my flash and re-flashed it to "r" in about 5min.

buy the Hymee Scanalyzer and check it youself http://www.performancedesign.com.au/

Ditto to what Zoom and Expo1 have said. This is an old topic but you are new so... a few practical observations.

I drive 5 miles each way to work and in the dead cold of winter get 14 to 15 MPG. In the spring, summer fall, I get 16 to 17 on my commute. max speed is ~50MPH. If that is similar to what you do, it is a comparison point. On the a tank full long trip I will get 21 to 23 MPG - just depends on whether I keep it below 80 and avoid WOT fun time.

Make sure your tires are pumped up...I find that 35psi cold while a little harsh, helps eek out better mileage.

Welcome to the RX-8, mines a late model 05 and it gets 11 to 12mpg city driving and the dealer has told me this is normal. BS

There is a problem with some RX-8 relating to poor fuel mileage and Mazda refuses to fix it. If you don't believe me see these links.

https://www.rx8club.com/rx-8-discussion-3/%2A%2A%2A-doubled-my-mpg-%2A%2A%2A-89281/

https://www.rx8club.com/rx-8-discussion-3/17-500-serious-mileage-change-89946/

Mazda is unwilling to stand behind this car and if you get a bad one your SOL.
Mazda just doesn’t care!