EagleTribune.com, North Andover, MA

March 31, 2013

New tire must match the old

Under the Hood
Brad Bergholdt

---- — 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.

It’s a good idea for all-wheel-drive vehicle owners to include their full-size spare with each tire rotation service so it remains the same size as its siblings. In the event a tire becomes injured and needs replacement, the spare becomes the replacement and the new, slightly larger tire goes to live in the trunk. Maintaining proper tire inflation pressure is also important.

For a great source of additional information on this topic, go to TireRack.com and click “Shopping Tools” in the top navigation bar, then click “Tech Center.” Select “Matching Tires on Four-Wheel Drive and All-Wheel Drive Vehicles.” You’ll also find very well-written articles on dozens of tire, wheel, brake, suspension and lighting topics.

My 2009 Lincoln MKZ with 35,000 miles is in great shape with no problems. I like to change the brake fluid every three or four years. Am I safe doing this myself —I have plenty of experience — or do I need to take it to the dealer because of the antilock brake system??

This is a good move. Because of complex fluid paths within the ABS brake system, Lincoln recommends pressure bleeding and refilling the system, as opposed to manual bleeding. The needed tool can be found for $100-$150, and the process is fairly simple for folks who can twist a wrench

Be sure to follow published procedures and use high performance DOT-3 brake fluid that meets or exceeds WSS-M6C62-A or WSS-M6C65-A1 specifications.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood@earthlink.net; he cannot make personal replies.