New wheels - new Speedcal settings needed?

stangGT97 · Nov 13, 2009

I just bought new 18" FR500s, with 285/35r18s for the back. My old stock tires are 245/45r17s. I've worked out the overall tire height
New: 285/35r18 = 25.85" tall (((285mm*0.35)*2)/25.4(mm/inch) = 25.85")
Old: 245/45r17 = 25.68" tall

That is a little less than 3/16" or 0.66% increase in wheel/tire height. That isn't enough of a change to warrant a recalibration of the speedometer, is it?

ChillPhatCat · Nov 13, 2009

It'll just make your speedo off by maybe -4 mph @ 80 (reading slower than actual speed).

I did the calculation on the height difference before... I forget the exact percentage though. Once you do it I'd suggest finding a handful of those radar guns on the side of the road and drive by at a certain speed and see how off it is. As you go faster the difference will be greater, since it's the percentage change in the tire circumferance that matters.

wmburns · Nov 13, 2009

Holy math class Batman, I think we have a problem.....

It's the case of the misplaced decimal point.

I agree the percentage difference is 0.662%. However, I think the error at 80 MPH is 0.53 MPH. Or around one half of a mile per hour.

In order to be off by 1 MPH, you will have to go 151 MPH.

stangGT97 · Nov 13, 2009

wmburns said:
Holy math class Batman, I think we have a problem.....

It's the case of the misplaced decimal point.

I agree the percentage difference is 0.662%. However, I think the error at 80 MPH is 0.53 MPH. Or around one half of a mile per hour.

In order to be off by 1 MPH, you will have to go 151 MPH.

OK you've independently confirmed the math I did. I won't bother looking for new dipswitch settings then. Thanks!

ChillPhatCat · Nov 14, 2009

I actually did no math for this calculation I was going off of what I remembered... so my memory must not be what it used to be. I guess I should have just redone the calculation... oh well.

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