17inch wheels MPH vs 15s.

[BIONICQ45]

Anyone know if there is a significant diffirence in mph with different wheel sizes? I know someone posted a calculator from a Miata site I think that calculated the change in mph reading as the wheels size changed. If anyone can remember that site or know a calculation let me know.Thanks.

[maxnix]

Speedometer and odometer differences are negligible as long as overall diameter is OEM designed 26". Hence all the discussion about correct tire sizes in previous posts.

[Checkered-Member]

Hence all the discussion about correct tire sizes in previous posts.

harsh....

this belongs in the suspension forum...

[lessthanjakejohn]

It depends, 15 v 17 is irrelevant when talking about speedometer readings. WHat you are worried about is OD, or the diameter of the wheel and the tire together. If this is bigger or smaller than the stock size (maxnix says its 26") then you will have speedometer error.

Perhaps a way to calculate this would be to take the OD and divide it by the stock OD. Then multiply the quotient with the MPH.

[Jeff Williams]

I use 17" x 8" rims on my Q, and I have been using 225/50/17, I have had 245/45/17, but went to 255/45/17, on the last set. These are a little wider, and closer to the actual diameter.

Actually Rotations per mile are what you are looking for. I think the number is around 775

63,360" per mile, and the circumference is the diameter x 3.14159

26 * 3.14159 = 81.68134

63360 / 81.68134 = 775.697 Rotations per mile

Look for a load rating of 98, and a circumference in this range, and you will be fine, o nthe speedo.

[AZhitman]

Chris: What size tires did you put on your 17's?

That's all we need to give you a good answer.

[lessthanjakejohn]

225/45 / R17 255/40 / R17those should be 25"

Perhaps he has already put tires on and wants to know the accuracy of his spedometer?

This site should help http://gs.tolan-hoechst.com/tirecalc.htm

[AZhitman]

John - That won't help. Our diameter is (I believe) 26.1".

That's why I wanted to know what size tires Chris bought.

[lessthanjakejohn]

doh! I misread maxnix's post for 25 instead of 26... oh well

I got the info from http://www.toyo.com

[SmithSR]

http://www.nissaninfiniticlub....43719 find the formula for indicated vs. actual speedo readings.

OD is critical, and can be found mathematically by adding the rim diameter and the sidewall heights together.

for example: if you were considering a 245/45-17(OD is 25.68")17" wheel (convert it to metric) 45% of 245mm multiplied by two(for both sidewalls)

so

17 x 25.4 = 431.8mm245 divided by 45 = 110.25mmadd both sidewalls...

431.8 + 110.25 + 110.25 = 652.3mm OD

Convert metric to standard: 652.3 divided by 25.4

25.68" OD for 245/45-17 tire

[SmithSR]

225/45 / R17 255/40 / R17those should be 25"

Perhaps he has already put tires on and wants to know the accuracy of his spedometer?

225/45-17 OD is 24.97"255/40-17 OD is 25.03"255/45-17 OD is 26.04"(now THAT'S a big meaty tire!!)225/50-17 OD is 25.86"235/50-17 OD is 26.25"

PHIL:)

[911/Q45]

Just to add to the complexity, different brands have different ODs in the same size! Tire Rack has specs for all they sell.

[AZhitman]

AND to make matters worse, some tires are different diameters FROM THE SAME MANUFACTURER. 2 of my Yoko's were .3" off (true story!) from the other 2.

[BIONICQ45]

Gregory, mine are 235/45/17.

[lessthanjakejohn]

according to site above, the error will be -3%

[Jeff Williams]

Gregory, mine are 235/45/17.

I had those, for a few weeks, and the ride was a little too harsh. They were Continental Sport Contacts. I went to 245/45/17 Continental Sport Contacts, then went to 225/50/17 Bridgestone Potenza RE92's Then I went to 225/50/17 NITTO NT455 Etreme Performance, Now I have the 255/45/17 Sumitomo HTR+

The Nitto's handled the best. The Sumitomo's ride the best, and are the quietest. I might have lost a .01G or so. The sidewalls are a lot taller, and absorb a lot of impact. With the 8" wide wheel, they loof FAT, & PHAT. :ylsuper