Tire size fitment
 

Hey i'm gettin' some 17" for my pro and i'm gonna put either 205/40 or 215/40 on them.
I checked out the tire size calculator and the 205/40 tires with the 17" rim would give me the same speedometer reading but the 215/40 tire would give a slightly slower reading
The results said:
When your speedo reads 60mph, you are actually travelling 60.8mph

Is that a bad thing?

0.8mph is so slight that it really won't matter.

ok thanks man
i didn't want to screw anything up and i'd prefer to put the 215's instead of the 205's

It was my understanding that the width does not affect the speedo, it's the aspect ratio (the height of the sidewall). So as long as you are maintaining the 40 part of the numbers with a 16" wheel, it should all be the same. Take a look at the Plus One concept of wheels/tires, ie if stock is 15" with 60 series tires, then you can go to 16" with 50 series with no change in speedo readings. This also works with other wheel size increases.

well yeah that makes sense.
i guess the tire size calculator isn't so accurate.