I replaced the OEM Pirelli P3000s (175 65 15) with Pirelli P5 (205 60 15). My fuel economy dropped 12% (35 mpg to 31.5 mpg). I expected some drop due to the heavier tires but this seems excessive. I presume (although I haven't been able to measure) a commensurate drop in acceleration. Any thoughts/ideas?

 

http://www.miata.net/garage/tirecalc.html

according to the tire size calculator (link above), you should have gotten 205/55-15 tires. the tires are probably much heavier, but someone else will need to comment on how much that would affect mileage.

 

The tire radius issue solves 3% of the fuel economy question. Now for other 7-9%...

 

The 30mm increase in width is what is really killing you... MPG is a non linear scale anyway, thus the drop isnt going to be as noticeable as you really think.

 

if you want mileage why would you get a tire that 30mm wider and taller? Stick with 175 and inflate it to the higher psi reccomended. But if its performance (wider more traction) just try a good sticky tire in 195 or 205 but a lower profile like 45 or 50 series. But performance is always a trade off for mileage. (most of the time)

 

Yeah, you can have tires like the ones I used to have, bad performance and bad mileage.