Car Talk Change to slightly thinner snow tires won’t impact speedometer, odometer
ē Q. I traded in our old Prius for a 2012 Prius last spring. Iím 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. Whatís your take on this? Is there a conversion formula? This would be a tough story to convey if I were stopped for speeding. Thanks. ē
TOM: The conversion formula is ďfuggedaboutit.Ē 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 (thatís the 15) and the same aspect ratio (thatís 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 thatís different is the section width (thatís 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 ó a difference of less than half an inch.
TOM: Width doesnít affect the speedometer or odometer. Only a tireís diameter can do that.
RAY: Now, since a tireís 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 arenít identical.
TOM: My math is not good enough to calculate how much error that tiny difference in diameter will cause in your speedometer, but itís a difference thatís so minimal, itís hardly worth thinking about.
RAY: Itís certainly not worth my brother breaking out his childhood abacus ó the one Confucius gave him.
TOM: And in fact, ďthinnerĒ 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, use your old snow tires, as long as theyíre safe. And since I doubt youíre doing a lot of drag racing in the Prius anyway, donít worry too much about any minor speedometer error.
ē Q. Winter is coming, and I have two gallons of oil/gas mixture (40-to-1) for my weed whacker that I donít 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. Thanks! ē
RAY: You live in Cleveland? You may want to just hang on to the gas-oil mixture and buy yourself a snow blower.
TOM: Actually, itís fine to use it in your car. Itís a relatively small amount of oil (1 part oil to 40 parts gasoline).
RAY: It wonít harm the engine, the fuel-injection system or the catalytic converter.
TOM: Itís not great for the environment, but, presumably, you would have burned it in your weed whacker anyway, so itís doing no more harm being burned by your car.
RAY: Most of my brotherís 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 chainsaw, your portable generator or your deluxe gas-powered butt-scratcher.
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. Thatíll minimize the concentration of oil and ensure that no harm is done.
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