Quote:
Originally Posted by krousdb
I would have agreed with you if you had said:
"more air pressure added to an underinflated tire DOES slightly change the diameter of a tire."
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i can see your point there, but to continue splitting hairs we have to determine what "underinflated" means. my tires are max rated for 44 psi, yet the (auto) manufacturer's suggested pressure is 32. below which of these numbers is the tire "underinflated" as would affect diameter?
if there is an upper limit on the amount of "stretch" in a tire's design, i bet it's at the tire manufacturer's maximum, not the automaker's. in which case, since most auto manufacturers ratings are significantly lower than the tire's max rating, most cars may see a FE benefit from both slightly larger diameter and lower rolling resistance by going with higher pressures.
i suspect there's a not insignificant diameter difference between the recommended 32 and the 48 i'm running in my tires. note my earlier comment: i measured a 2 mm
radius difference between 38 and 48 psi - though admittedly the only way to know for sure how much of that is diameter change as opposed to "de-bulging" of the sidewall would be to run a controlled distance test as jangeo suggests.
also, if there is a difference between 32 and 44/48, i suspect it's not so much from "slip" (which i would expect to see at much lower than 32) as it is from an actual (if small) diameter/circumference difference.
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