LITTLE ROCK — 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.
- Paul
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 narrower 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 narrower 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.
DEAR TOM AND RAY: Winter is coming, and I have two gallons of oil/gas mixture (40-to-1) for my weed whacker that I don’t want to save for next year or try to dispose of. Is it safe to add this mixture to an almost-full tank of gas in my 2004 Honda Accord? I use 87 octane and live in Cleveland. - Jeff
RAY: You live in Cleveland? You may want to just hang on to the gas-oil mixture and buy yourself a snow blower, Jeff.
TOM: Actually, it’s fine to use it in your car. It’s a relatively small amount of oil (1 part oil to 40 parts gasoline).
RAY: It won’t harm the engine, the fuel-injection system or the catalytic converter.
TOM: It’s not great for the environment, but, presumably, you would have burned it in your weed whacker anyway, so it’s doing no more harm being burned by your car.
RAY: Most of my brother’s cars burn more oil than that on their own, due to age and/ or disrepair. In fact, some of them burn more oil than gas.
TOM: Keep in mind, you also can save the mixture for next year. Gasoline generally is good for at least a year, these days. You can make it last even longer by adding a small amount of gasoline stabilizer, like Sta-Bil.
RAY: And if you keep it, you also can use it in your lawnmower, your chain saw and your portable generator.
TOM: But if you do decide to power your Accord with this mix, I would do it over several tankfuls, rather than dumping the whole thing in at once. That’ll minimize the concentration of oil and ensure that no harm is done.
Click & Clack (Tom and Ray Magliozzi) dispense advice about cars in Car Talk every Saturday. Email them by visiting cartalk.com
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