That's a tiny change... but it got me thinking about how much tire pressure and wear affect revolutions/mile, and so speedo/odo cal.
Per Tire Rack, Michelin makes the OEM tires with 9.5/32" of tread depth.
- I'll calculate using unloaded dimensions; there's a consistent 3% difference between loaded and unloaded radius.
- Assuming a 3/32" threshold for "worn out," one expects a 6.5/32" change in radius over life, or a 13/32 change in diameter, 0.406"
The result is a 1.6% change in static diameter over life.
Michelin rates the OEM tire at 1433 lb.@51psi. Tread width is fixed at 8.2". Contact patch length will vary with pressure.
- from Pascal's Law
Force = pressure x area, so contact patch area = force / pressure, or 1433 / 51=28.8 sq. in.
- assuming square corners (a bad assumption), contact patch length = area / tread width
- the hard part is calculating the radius change due to a "flat spot" the length of the contact patch
Loaded R = static R x (1 - cos(x)),
where x = asin(contact patch length / static diameter)
The result is a 0.3% change in rolling diameter going from 51psi to 38psi., up to 0.6% if you go down to 30psi.
I have raised my tire psi from 45 to 50 and plan to move front to back the tires, Rears have more tread and will see what effect that has on GPS mileage. The difference was 1.38% on my record breaking trip. I'm thinking it might go to 1.5%.