Interesting post from a guy here at GTA:
I never thought that millimeters would make that much of a difference in fuel consumption!
It doesn't! I thought it was fairly obvious..the difference is not
actual. The difference is only
indicated because you are using an instrument (the odometer) that relies on a constantly variable quotient (tire wear).
That's also why I suggested using a GPS, because the results don't lie.
This example can also be shown using mathematics. Simply mark a bald tire and the ground at exactly the same point and rotate it one revolution. Mark the pavement again. (FWIW, it might be easier to turn it 10 revolutions which will help fend of slight differences, and will result in a divisionally more accurate conclusion).
Then do same test with brand new tire. You will see the new tire has gone farther in one revolution than the bald tire did. Make note of that measured extra difference for use later in the calculation. Now measure the entire distance you rolled both tires and divide that measurement into 1 mile (assuming we are using miles per gallon..if not then the result can still be adjusted using the math equivalent). Call that calc a and b. Now take the difference between bald and new and multiply it by the result you got in a and b. The final equation will literally show you the discrepancy in distance travelled. The constant here is the measured 1 mile. If you turn the new tire as many times as the bald did in 1 mile, you will see that the new tire travelled well past that 1 mile mark. And there's your difference between a new and used tire.
FWIW, if you want to compare this scenario on a level that JZ67 would consider anal+, even tire PSI can have a slight effect on overall tire circumference. Higher PSI's stress tire to a greater circumference than lower PSI's. As does a brand new tire will expand a little during its first few days of use thereby lowering the original PSI a little.
Going for anal++ if said tire was constantly turned at 150 mph vs 50 mph, the extra speed will (probably) expand the tire into an artificially created greater circumference thereby skewing the results even further. I said probably, because we are talking generally here, since we do not have a specific brand/model of tire on a lab-controlled device to spin it up to a high speed and video the change in profile due to the centrifugal forces exerted on the tire. Due to tire construction it is possible for a tire to take on a lower profile under highspeed stress, but it is unlikely. Other variables which could be included are whether the tire is weighted during the test..etc etc etc. The parameters are vast if you go this anal.
Anyway, hope this helped illuminate what is actually happening for you a little. The bottom line is...your actual fuel economy is the same. The only time it would change due to tire wear is if we added into the equation that a worn tire raises RPM ever so slightly at the same set speed as a new tire and that change in RPM may or may not allow engine to operate at a more efficient rpm range, thereby possibly affecting fuel mileage. Let's call that anal+++.
edit- I could have added another parameter ( a very significant one unless you live in the prairies) and that is the effect that cornering has on your odometer reading. But instead of going there..gonna call it a day here..
