Revolutions per mile 12R 22.5 XZE Michelin

[Melbo]

I would like to recalibrate my speedometer

I am adding another set of magnets so it will double the pulse count -- I'm hoping it will help with the cruise and the speedometer

The cruise seems to work fine but the speedo seems slow compared to the GPS

I was wondering how many revolutions per mile to enter when I am setting up the new count in the speedometer

Thanks

Melbo

[Lin]

I do not know this offhand, but it shoiuld be available on the manufacturer's data sheet online along with the inflation specs.

[HighTechRedneck]

Mel, specs say 486 rpm.

http://www.michelintruck.com/michelintruck/tires-retreads/new-tires.jsp

(you have to choose the XZE* from the list to see the 12R22.5)

[Melbo]

Thank You

Melbo

[usbusin]

Just curious; how much do the revs/mile change from a new tire to one worn down to the "wear bars"?

As the tire wears, you have a smaller diameter, therefore more revs/mile.  How much does this affect speedometer accuracy?

[HighTechRedneck]

Good question.  Let's figure it out using the XZE as an example.

Specs say:
- Total diameter = 42.6"
- Tread depth = 22/32"
- RPM = 486

This won't be entirely precise because of the flat portion of the tire while loaded, but the percentages should be about right I would think.

The circumference when new would be 42.6" x pi = 133.8"

The generally accepted minimum acceptable tread depth is 2/32"  So the consumable tread would amount to 20/32".  Multiply that by 2 for the total change in diameter and that is 40/32" or 1.25"

A change of 1.25" x pi = 3.9" change in circumference.

Now calculate the percentage change.  3.9/133.8 = .029 = 2.9%  To make it simpler we can round it up to 3%.

So with the tire worn down to 2/32" tread remaining, the circumference would be 3% less, the speedometer would be 3% fast, and the RPM's would be 3% higher.

At 70mph actual speed, your speedometer would read 72mph.

[pvcces]

It looks a whole lot more like 3% to me. I think that you subtracted diameter from circumference.

For what it's worth.

Tom Caffrey

[HighTechRedneck]

Good catch Tom.  I failed to multiply the change in diameter times pi to get the change in circumference.  I corrected it now.

[RoyJ]

Just to add a bit more, I think we'll see a bit more than 3% in reality, as we're loosing 20/32nd from the loaded radius, which would be less than the 21.8" unloaded radius of the tire.