Tell me if I'm crazy, but here's a back of the envelope calculation: I just got new tires, after my old tires wore down to the warning bars. The new tires are the same size as the old tires, set to about the same pressure 42/40. I am noticing about a 3%-4% decrease in MPGs on the computer--from about 50 to about 48 computer measured, about 5% above reality. So, I figure I wore off about a quarter inch of tread, that's a half inch of diameter, that's about 2% of 25 inch diameter tires. So my new tires need 2% fewer revolutions to go a mile--but since no energy is free, it requires about the same fuel expenditure to go the mile. However, the car records me as going a 2% shorter distance on that fuel. (I assume that the car manufacturers gauge the odometer to new tires? Right? So in fact the new tire MPG should be closer to the truth than the old tire MPG?) The other 1 or two% difference I put down to what some of you have told me: New tires have slightly higher rolling resistance than worn tires. Also, my new tires, Continental ProContact with EcoPlus, have higher rolling resistance than my old Goodyear Assurance Fuel Max. Are these estimates reasonable? Is the theory OK?
Actually a worn tire can have 20% less rolling resistance than the same tire when brand new. The fuel Max is also more efficient in general. The Procontact is a much better tire though.
Based on previous discussions here (search for 'neutral axis') and some experience, I'd say no. The tire flattens where it contacts the road, so its effective rolling diameter is different than its apparent diameter. What you really want to know is its rolling circumference, which is controlled mostly by the length of the steel belts just beneath the tread, and only weakly altered by tread depth and inflation pressure. Look up the tire RPM -- Revolutions Per Mile. If I read these right, your old tires were rated 855 revs per mile. The new ones are just 848, an 0.8% difference, which corresponds to an effective rolling diameter 0.2" taller. The rest of the difference should be from the different rolling resistances of the different tire models, plus the increased resistance of new tires.
So what you are telling me is that the old tires were about 855 revs per mile pretty much both when new and used. Sounds counterintuitive, but I can almost visualize it when I think about the steel belt length. Thanks!
The tire revs per mile isn't exactly the same when new and old, but reality is much closer to this than to the model of a rigid circle with a wearing layer of tread. One way to look at it is that as the tread flexes where it flattens at the contact patch, the rubber mushes around far more than do the steel belts. Another poster brought in the 'neutral axis' concept commonly used with other flexing structures. Since the steel belts are much stiffer than the rubber in which they are embedded, the neutral axis lines up very close to the center of the belts.
I noticed when I purchased my Energy Saver tires three years ago I got a 2-3 mpg increase over the Yokohama Avid tires which had over 10,000 miles on them. Tire size and air pressure were the same for both.
