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
I do not know this offhand, but it shoiuld be available on the manufacturer's data sheet online along with the inflation specs.
Mel, specs say 486 rpm.
http://www.michelintruck.com/michelintruck/tires-retreads/new-tires.jsp (http://www.michelintruck.com/michelintruck/tires-retreads/new-tires.jsp)
(you have to choose the XZE* from the list to see the 12R22.5)
Thank You
Melbo
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?
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.
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
Good catch Tom. I failed to multiply the change in diameter times pi to get the change in circumference. I corrected it now.
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.