Calculating wheel revolutions per mile - which spec to use?

[mikes2017gt]

My tire/wheel setup will change the overall diameter of the wheel vs. stock. I want to recalibrate the speedo, which I can do with Ford Performance ProCal flash tool that came with my PP2. In the software there is a user-settable entry for Tire Revolutions Per Mile.

The manufacturer states my new tire makes 772 revs per mile. See here: http://www.continentaltire.com/product/extremecontact-sport-28535zr19-99y/?

However, one of those "wheel offset calculator" sites and the math say that number should be 751. What gives? Is the manufacturer wrong? Well, turns out that wheel offset website has a note that says this:

Revolutions per mile of a mounted tire under load will generally be about 3% more than the calculated number. Individual tire specs may vary from the calculations. However, you can choose from the populated list of tires to view the manufacturer specs for each tire. Specs are based on an inflated tire mounted on the 'Measured Rim' size and NOT installed under the weight of a vehicle.(If you measure the tire on your vehicle from the ground up, it will be shorter than the manufacturer diameter spec.) Tire widths will vary slightly depending on the rim width. Generally the tire width will change 2/10" for every 1/2" in rim width when mounted within the rim range. Tire diameter is not significantly affected by rim width.

And that checks out. 3% of 751 is 22.5. 751+22.5=773.5, which is close enough to Conti's spec of 772 revs per mile.

So which number should I input as my tire revolutions per mile? And I thought this was going to be cake...

Thanks in advance!

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[Brazos609]

I can say they 285/35R19 Continental DWS06 tires I got were considerably smaller in diameter than the 285/35R19 Nito NT555G2 tires I had. Smaller diameter, more revolutions per mile. I would plug in the 772 and check it with a GPS app.

 

[BmacIL]

Use what the tire manufacturer says. They factor in being mounted on a rim, at recommended pressure and the size change that causes vs unmounted.

 

[BmacIL]

I can say they 285/35R19 Continental DWS06 tires I got were considerably smaller in diameter than the 285/35R19 Nito NT555G2 tires I had. Smaller diameter, more revolutions per mile. I would plug in the 772 and check it with a GPS app.

Erm, I am thinking one was not actually 285/35R19. Tread width and section width varies by brand for a given size, but diameter is very, very consistent. Both are 26.9" OD.

Top: NT555 G2, bottom: Conti DWS06

 

[mikes2017gt]

Thanks very much, BmacIL. Appreciate your advice.

Use what the tire manufacturer says. They factor in being mounted on a rim, at recommended pressure and the size change that causes vs unmounted.

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