Does this affect my mileage on my car?
 

My rx-7 has 3 different brands of tires on it. 2 bridgestone potenza r930's on the driver's side, a michelin energy plus on the front passenger side, and a michelin pilot sport on the passenger rear. My car is getting about 8 mpg, and it's automatic. I changed everything on the car except the fuel filter, auto trans fluid, and auto trans filter from fear of something breaking.

The different tires shouldnt have a noticeable effect on mileage. If they do have any effect, I wouldnt think it would be that big. And autos actually get less gas mileage than manuals, because it takes more power for the tranny to switch gears. You changed spark plugs, wires, air filter, all the normal stuff?

yup
 

I've done the whole shebang to the car, except the filter and fluid for the tranny and the fuel filter. But yeah, I'm not really interested in power, I just want to maintain my car as a fully stock car. I just want to improve the mileage! I cannot afford a car that has 8 mpg!!!!

Make sure the tires are inflated to spec. Underinflated tires can reduce your mpg's. However, I severely doubt that much.

Check for codes:
http://www.1300cc.com/howto/how2/codes.html

It all depends on acoustic properties of the different tires. If one brand of tire's resonant frequency is mathematically a whole fraction of another brands, the acoustic shock waves tend to collide just forward of the rear tire, basically created a positive pressure air wall that will slow you down, and results in horrible gas mileage...

OK, I can't hold a straight face much longer- at what point in the above did you start telling yourselves "bullshit" :)

Check your compression, man...If she's getting old & tired, she'll eat more gas...

Won't hurt to check your O2 sensor input while driving, either, to make sure she's going into closed loop on the freeway. You can do it from the ECU, using this:

https://www.rx7club.com/2nd-generation-specific-1986-1992-17/troubleshooting-your-car-ecu-340578/

thx
 

Thanks everyone, I'll try checking the codes, pressure, and the o2 sensor right now!

Really? ;)