One of the first things I will do when the Mini S arrives is to swap out the run flats for standard Michelin tires. I have read enough about them on the BMW forums and this one to know that I don't want them. I was considering increasing the size to 215. It will throw off the speedometer by 1 MPH. Any thoughts?

 

I went from Dunlop Runflats 205/45/17 to Bridgestone 960s 215/45/17, and the increase/decrease is from 853 revs per mile to 847 revs per mile. That's 0.7%. So unless my math is wrong, you'll only be off 1mph at max speeds.

I think I lost a little mileage due to the larger footprint, but not in speedo error.

 

Most speedos are off by at least 1 mph anyways, so I think it's rather moot. The natural upsizing from 205/45 would be to 215/40's though. Also note that different tire brands of the same size does not mean they are exactly the same size when you measure it, due to different types of tread design and construction. Last thing, regarding revolutions per mile, while it may be fine to do theoretical comparisons, it'll change over the life of the tire simply as the tread wears down.

 

Maybe, but if the number is accurate for a new tire, it will give you the most accurate result in comparing tires. Obviously you have to take into consideration tire wear, brand, inflation pressure and other factors.

Why do you say that "The natural upsizing from 205/45 would be to 215/40's"?

 

OEM Dunlap SP Sport 01 DSST 205/45ZR17 runflats revs per mile: 853

Michelin Pilot Exalto PE2 205/45ZR17 non-runflats rev/mile: 856 = 100.35% of OEM

Michelin Pilot Exalto PE2 215/40ZR17 non-runflats rev/mile: 875 = 102.58% of OEM

Michelin Pilot Exalto PE2 215/45ZR17 non-runflats rev/mile: 846 = 99.18% of OEM

The 215/45 are 0.82% oversize.
The 215/40 are 2.58% undersize.

Looks like 215/45 are closer to OEM.

 

Granted a 10mm increase in tread width is minimal, increasing tread width without decreasing sidewall height would increase overall diameter. But as you pointed out, the overall increase in this instance will be minimal.