Since I switched from 205/45/17 (Pirelli RF) to 215/45/17 I've been getting worst mpg... I've tried Falken FK452 and Toyo PX4 now. Usually I got 300-310 miles with the 205/45/17, but now I'm getting just 275-285 with the 215/45/17...(full tank) Same driving conditions, just tires changed.
Does anybody have had this issue?... How many PSI do you usually use on your tires?

 

Are you driving harder [more right foot] to test the new rubber? You'll probably lose a tenth of an MPG for markedly wider tires and the road and aero drag they'll cause, but such a small tire size change shouldn't yield a noticable change. A bigger factor would be too-low tire pressure. You should be running ~36 PSI front and rear.

 

Thanks for the reply, but as I said, same driving conditions... I may start considering that is a virtual number...the diference in mpg is not real at all...

 

Is there a O.D. difference in the two wheels? I'm not sure how the OBC calculates miles, but perhaps that throws it off?

What about your tire pressures? What are you running?

 

Yes, the outer diameter of the new tire is bigger, by 1.5%. That means you are actually going 1.5% farther than your odometer says. And faster than your speedo says, by 1.5%.

Also, the wider tires probably have more "road resistance" than the originals.

But that doesn't acount for all the difference you are seeing.

BTW- I get worse mpg in the summer, when the AC works so hard, my mpg drops to 29-30, for mostly hiway miles on my 2006 S.

Here's a tire size calculator:
http://www.miata.net/garage/tirecalcold.html

 

Thanks for the Link! Very useful!
So, do you think it worths to go back with the 205/45/17?

 

Your choice.

Stock size gives you stock performance, stock mpg.

see
https://www.northamericanmotoring.co....php?p=1634626

Tire selection is larger in 215/45-17

 

Is there any thing one can buy/add/rewire/software, etc that can correct for varying tire sizes?