Speedometer accuracy with different wheel size?

[Cavman]

Am scheduled to put on a new set of 20" rims (214s) on my 35i Premium in early March and wondered if the larger wheel diameter would throw off the accuracy of the speedometer since the car came with 19s.

I apologize if this is a dumb question.

[kakalika]

Not if you keep the overall diameter the same.

Here is a wheel size calculator I always use. It will tell you the difference in sizes and the effect on speedometer.

http://www.1010tires.com/tiresizecalculator.asp

[ard]

Skip the calculators. (They are fine, but not necessary and not as accurate as looking at the exact specs)

Look up the tires you currently have on tirerack, look at the specs. Look for the "revs per mile" number.

Then look up the new tire you will be using- compare those two specs. The % difference will be the % change in indicated speed.

it is well established that BMWs tend to indicate higher than actual...so you can go up a few percent in size and still have an 'accurate' speedo.

A

PS (Think I got that right.... )

[JCL]

Well, to be technically accurate....

When you go up a bit in diameter, the revs per mile drops. So, the % difference in revs per mile will be the negative of the % change in indicated speed

Just to be precise

[ard]

I figured it was a coin flip....

So if your new tire is a more revs per mile, it is a smaller tire.... (smaller tire needs more revs to cover the same one mile).... take that new tire, and when on the car at the same speed as before it will give you a higher indicated speed reading....

A

[Cavman]

Thank you guys. Great info. Very informative.

Seems like a move from 19" to 20" is about a 0.7% increase in revs per mile (19" OEM tires are 716 revs/mile and 20" OEM tires are 721 revs/mile).

Therefore, if I am doing the math correctly, with the 20" tires and the speedometer reading 65mph, I will actually be traveling 65.455mph.

This doesn't take into consideration any "padding" that BMW has apparently added to our speedometer.

Thanks again.

[finagle69]

Originally Posted by Cavman

Thank you guys. Great info. Very informative.

Seems like a move from 19" to 20" is about a 0.7% increase in revs per mile (19" OEM tires are 716 revs/mile and 20" OEM tires are 721 revs/mile).

Therefore, if I am doing the math correctly, with the 20" tires and the speedometer reading 65mph, I will actually be traveling 65.455mph.

This doesn't take into consideration any "padding" that BMW has apparently added to our speedometer.

Thanks again.

That's a half truth.

Not all tires are the same OD for the same wheel size. This is why you can buy 315/25/20 or 315/30/20 or any other combination. The middle number is a percentage of the first number, so the higher the percentage, the larger the OD will be. Thus giving you an even lower rev/mile.

This is why the calculators can actually be helpful.