Dear Tom and Ray:
I traded in our old Prius for a 2012 Prius last spring. I'm getting ready to put on the snow tires that I kept from the old car. Those tires are P185/65 R15, and the all-season tires on the new Prius are P195/65 R15. The dealer said the snows will work fine, but the slightly different tire will affect the speedometer and odometer. What's your take on this? Is there a conversion formula? This would be a tough story to convey if I were stopped for speeding. Thanks.
TOM: Paul, the conversion formula is "fuggedaboutit." The effect on your speedometer and odometer will be minuscule.
RAY: Your two sets of tires are almost identical. They have the same wheel size in inches (that's the 15) and the same aspect ratio (that's the 65), which is the ratio between the width of the tread (the section width) and the height of the sidewall.
TOM: The only number that's different is the section width (that's the width of the tire).
RAY: What does that mean? It means your old tires are a little thinner than your new tires. The new ones are 195 millimeters wide, and your old tires are 185 millimeters wide -- a difference of less than half an inch.
TOM: Width doesn't affect the speedometer or odometer. Only a tire's diameter can do that.
RAY: Now, since a tire's aspect ratio is a fixed ratio between the width and height of the tire, a thinner width means that, by definition, the height (and therefore diameter) must be a little smaller, too. So, the tires aren't identical.
TOM: My math is not good enough to calculate how much error that tiny difference in diameter will cause in your speedometer, but it's a difference that's so minimal, it's hardly worth thinking about.
RAY: It's certainly not worth my brother breaking out his childhood abacus -- the one Confucius gave him.
TOM: And in fact, "thinner" tires (with a smaller section width) often are recommended for use in the snow, because, while wider tires may ride on top of the snow, thinner tires have a better chance of cutting through the snow to the pavement and giving you traction.
RAY: So, Paul, use your old snow tires, as long as they're safe. And since I doubt you're doing a lot of drag racing in the Prius anyway, don't worry too much about any minor speedometer error.