We Might Need a Nobel Laureate to Solve This Problem

  • Ray Magliozzi
  • 8/22/2020

Dear Car Talk:

I've always loved the columns by you and your brother! You recently answered a reader who wanted to know why his Prius' mileage dropped after he got new tires. You suggested his new tires may have been "higher rolling resistance" than his original tires.

I agree that's possible. But I would also add a "measuring error." I do not believe the Prius is able to adjust the calculated MPG for tire wear (or tire size, for that matter). If my calculations are correct, 3/16 inch of tire wear decreases the circumference of the tire, and results in 2% "higher" MPG.

When you buy new tires, you "lose" that advantage. For most cars, you would hardly notice this because of their lower MPG, larger tires, less precise displays, etc. But we Prius owners are obsessed with our fuel numbers, so your reader noticed it. -- John

I think you have it backward, John. You've always hated the columns by me and my brother. Actually, I think it's the "mileage error" that works the other way.

When your tire's circumference gets smaller due to wear (or due to compression from excess weight in the car, or other reasons), you travel less distance with each rotation of the axle. That lowers the "miles" part of your miles per gallon.

Think of an extreme case. Let's say you had huge tires on your car -- tires that had a circumference of 1 mile. OK, it'd take you a year to fill them up with air at the gas station, but just imagine it for a minute. For every rotation of the car's axle, you'd go 1 mile. Wow. All else being equal, you'd get incredible mileage with those tires, right?

On the other extreme, if you had little, 1-inch roller-skate wheels on your car, the axle would turn and turn and turn, and you'd go very little distance, and get poor mileage. So, in theory, a larger tire should increase your mileage, not reduce it.

However, if you're relying on the car's internal computer to measure your mileage (rather than, say, mile markers and actual fuel measurements), you might be right, John. The car's computer may show decreased MPG on newer, larger tires.

Here's why. The computer is programmed to calculate distance based on how many times the axle rotates with the factory wheels and tires. So let's say Toyota calculated that it takes 1,000 turns of the axle for that Prius to go 1 mile. When your tires wear down, it takes less fuel to turn the axle 1,000 times. Why? Because you're not actually going as far.

So the car's dumb computer thinks you've gone a mile (even though you haven't), and it says: "Hey, we've used less gasoline to go a mile! Yippee!" Then, when you replace the tires and make the car's factory calculation correct again, you may actually see your car computer's mileage decrease a bit.

I would argue that, in real life, once you get into overdrive, you'd get better mileage with newer, larger tires. But we may need Nobel laureates in physics to resolve this for us with any degree of certainty. It's very confusing.

So, John, whatever you do, please don't write to us again with any more questions.

Todays Car-o-Scope

What the stars say about your car for 9/19/2020
Keep an eye out for important warning lights, only some of which will appear on your dashboard.
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