i heard that if u have different diameters in getting 18s with 225-40-18 then 17's with 225-45-17 it will messup your like speed,your engine, gass milage and stuff like that. is it true? because the diamater is bigger?

 

Changing your overall wheel diameter changes:

1. Your gearing (longer if larger, shorter is smaller)
2. Your speedometer reading (will read lower if larger, higher if smaller)
3. Your gas milage (through gearing as in #1. Lower is longer, higher is shorter)

Nothing else really gets affected besides that and handling characteristics.

 

www.miata.net/garage/tirecalc.html

 

with those two given tire sizes the difference is so small (0.1") in outer diameters you are not going to notice any differences at all.