The Rx-7 and 8 are Rotary "****le" engines and are known to be completely crap for fuel economy the best you could expect from the old Rx-7 was about 13mpg which was a shame because it was absolutely brilliant to drive, they where also known to drink around a litre of oil every thousand miles!
Not driven an 8, felt quite depressed when Mazda bought it out minus the turbo from previous models.
In the old classic wagon 333/329 I managed 347 miles on a tank, (crawled 140 on A roads and the rest on the M4) , I think I overtook like 2 cars on the A roads.. that was off boast with luggage and just me in the car.. driving spirited I managed to empty a tank in less than 140 miles
I don't even try in the blob its just not worth it.. as Anger says.. off boost till its warm
Think my average is around 210 a tank.
edit.. Please forum mods " w a n k l e" is not a swear word :P