EagleTribune.com, North Andover, MA


March 31, 2013

New tire must match the old

When a tire on my 2009 Subaru Forester was ruined, I tried to get it replaced under a hazard warranty. First the serviceman told me I needed to replace two tires because the vehicle is all-wheel drive and he couldn’t replace just one. Then they told me they had to replace all four. I think it was because they didn’t have the same type. What is the harm in replacing one tire that had less than 700 miles? They didn’t have the exact same model of tire, but it was the same manufacturer, size and speed rating. I didn’t want to leave the store with a $700 bill and three almost brand-new spare tires, so I bought the one tire and mounted it on the rear passenger side.

It’s true your Forester’s four tires need to be virtually identical in rolling circumference to avoid excessively wearing or damaging certain all-wheel-drive components.

Tire size, tread wear and rolling deflection, which is affected by tread type and inflation pressure, determine how many revolutions per mile each tire will make.

Subaru recommends keeping all four tires within a quarter-inch in circumference, or 2/32-inch tread wear. This is about three revolutions per mile. Mixing tire models, even of the same brand and size, could result in a differing circumference.

TireRack.com lists, as available from manufacturers, revolutions per mile for many tire brands and models. Try comparing your existing tire model to the one purchased. If the revolutions per mile is plus/minus 3, you should be OK. I’m assuming all existing tires had only 700 miles wear, with practically no loss of rubber

If specs are unavailable for your tires, find a straight, secluded roadway and mark each properly inflated tire at the twelve o’clock position with chalk or a piece of tape. Drive 0.1 mile and recheck position, then 0.2 miles. You’ll quickly get some data to project any differences in revolutions per mile.

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