Tire revs/mile and odometer calibration

[California98Civic]

Originally Posted by hawk2100n

I had been wondering what people did to calibrate the odo specifically on civics. I'll have to check my numbers now. I just replaced my 175 70 R 13s Prime Wells with a set of LRR Michelin Defenders 175 65 R 14 with aluminum rims from an 04 Civic hybrid.

Old were about 917 rev/mile
New are rated 905 rev/mile

wich gives a 1.31% difference

I always just use the stock calibration on my scan gauge for its numbers however my fuel econ numbers come from the stock odometer versus gallons pumped with no correction and no scangauge data. Perhaps I should do some analysis on my new and old setups to see if any big adjustments are in the cards for me. I still have the old tires so it would be easy. A 5.5% bump would be huge even if it is only an arbitrary number at that point with no actual bearing on consumption.

The revs/mile of tirerack are just corporate reporting. Worth testing for yourself in the real world conditions of your car and specific set up. The GPS I'm playing with has confirmed, almost, what ridewithgps.com showed me... so if you don't have a GPS, plot out your daily commute carefully on ridewithgps.com and compare what your OEM odo reports. I'd like to know, because I still find my calibration correction astonishing and hard to believe. I'm waiting for an error to appear.

[JRMichler]

I recently put a new set of tires on my truck. The old tires were not quite down to the wear bars. The revolutions per mile changed about 1%. There is also a tire pressure effect where increased pressure reduces the revolutions per mile. I do not know if that effect is significant, but it is definitely there.

[mcrews]

Originally Posted by JRMichler

There is also a tire pressure effect where increased pressure reduces the revolutions per mile. I do not know if that effect is significant, but it is definitely there.

it's a steel belt.....40 or 50 psi isnt going to change the diameter

[PaleMelanesian]

Originally Posted by JRMichler

I recently put a new set of tires on my truck. The old tires were not quite down to the wear bars. The revolutions per mile changed about 1%. There is also a tire pressure effect where increased pressure reduces the revolutions per mile. I do not know if that effect is significant, but it is definitely there.

Originally Posted by mcrews

it's a steel belt.....40 or 50 psi isnt going to change the diameter

Seems to me it could, if the lower pressure is allowing a flat spot. The smaller effective diameter of the flat spot leads to higher revs/mile. Increasing pressure can push that out closer to round, making the tire act like it's bigger.

[Fat Charlie]

I can see low pressure with a car sitting on top of it, allowing deformation and reduced diameter. Higher pressures really shouldn't stretch the steel belts past nominal size. I'd think higher pressures would increase resistance to deformation (=LRR) without an actual increase in diameter.

This doesn't account for people like Ford specifying really low pressures in their Firestones for a cushy ride, but if reasonable people set the spec, going to max sidewall or beyond shouldn't affect size.

[California98Civic]

Originally Posted by Fat Charlie

I can see low pressure with a car sitting on top of it, allowing deformation and reduced diameter. Higher pressures really shouldn't stretch the steel belts past nominal size. I'd think higher pressures would increase resistance to deformation (=LRR) without an actual increase in diameter.

This doesn't account for people like Ford specifying really low pressures in their Firestones for a cushy ride, but if reasonable people set the spec, going to max sidewall or beyond shouldn't affect size.

This makes sense, but I think Pale still has a point. If the OEM psi spec is low enough to allow deformation under the car's weight (which it does of course) then higher pressure might help the tire keep more of its max design diameter. If the car makers factor the deformation into their expectations for the odometer/speedometer, then wouldn't higher pressure produce fewer revs per mile? What am I misunderstanding?

[wolydotmatrix]

Does the outter diameter actually change? Does the tire just touch more ( or less) of the road. A ballons circumference will change due to its phisical properties. Does the total circumference change or does the tire just become more round?

[JRMichler]

Increased pressure causes the cords, even steel cords, to get longer. The exact amount is pretty small for steel cords. I did some rough calculations that seem to show the effect is too small to measure. But it is there.

Increased pressure will make for a smaller contact spot. I don't know if this changes revolutions per mile in a car tire.

I know that tire pressure changed revolutions per mile in a bicycle tire. I could tell the tire pressure from the odometer distance for a known trip.

[gone-ot]

Originally Posted by wolydotmatrix

Does the outter diameter actually change? Does the tire just touch more ( or less) of the road. A ballons circumference will change due to its phisical properties. Does the total circumference change or does the tire just become more round?

Q: Does outer diameter actually change?
A: Yes!

FWIW, the "loaded" tire diameter (what you'd use to calculate tire-revolutions per mile) is typically about 0.97 of the tires un-loaded diameter (my analysis of Michelin and Goodyear tire data)

But, the amount of diameter INCREASE from higher airpressure is MUCH smaller due to the constricting force of the steel belts (<0.03).

[mcrews]

you gotta love Mr. Wiki!!!!