Q: While I agree that tires are an important safety component, I mildly disagree that they are relatively low-cost (new tires on my 2003 Expedition set me back over $1,100 two years ago). Even selling three lightly worn tires wouldn't put much of a dent in the purchase price of four new tires. But my question is with your definition of "significant difference" in tire size and the stress it puts on driveline components. No doubt having a 16-inch tire mixed in with three 18-inch tires would cause many problems in a modern vehicle. But, using my Expedition as an example, its R20 tires are nearly 32 inches in diameter or approximately 1,024/32nds. Assuming that 3/32nds of tire wear would cause a 6/32nds decrease in diameter measurement, my worn tires would have a diameter of 1,018/32nds. My math is not great but to me that seems like the diameter of the worn tire would be about 99.4% of the diameter of a new tire. Does six-tenths of 1% really qualify as "significant difference"? I'm sure this difference might be huge in another scenario with very tight tolerances. But is it so in this case?

A: Fair question. Since you focused on percentages in your reference to tire wear, let's do the same for replacement tire costs. The original purchase price of a 2003 Expedition was between $35,000 and $40,000. The $1,100 you spent on new tires represents about 3% of the original vehicle cost — a very small amount for the components most directly influencing the safety of you and your family.

Your point about tire wear is valid, although your numbers aren't quite accurate for the Acura MDX in the original question. The original tire for that vehicle is a P255/55-18 with a diameter of 29.06 inches. Tire wear of 3/32nds of an inch reduces the diameter by 6/32nds, leaving the tire at 28.87 inches. That's about a 1% change in diameter. That means the worn tire will rotate about 10 fewer times per mile of travel.

Is that too much driveline stress on a 4/AWD vehicle? It would be on mine, but you be the judge. According to Tire Rack's Web page, Audi wants the tires within 4/32nds of remaining tread depth. On the other end of the spectrum, Subaru wants all four tires within 2/32nds of each other in remaining tread depth.

Considering the $25-$35 (also from Tire Rack's Web page) to shave a tire, I just don't see a significant benefit in purchasing and shaving one new tire.

Q: I inherited a 2005 Buick Century over a year ago with only 8,000 miles on it. Currently it has a little over 16,000 miles. It always has a robust start on a cold start but if I am running errands and making several stops, sometimes it has all it can do to turn over when started. It always does, so far, but just barely at times. I have had it to the dealer twice and an independent shop but they can't find anything wrong. No warning lights come on and everything checks out diagnostically. I only drive it a few times a week. The dealer said I should try driving it more often. Is there any insight you can give me?

A: If the vehicle cranks over strongly on a cold start after sitting for several days, it should restart after an errand just as well. The most common causes of slow engine cranking are poor connections in the starter circuit or the starter motor itself. Carefully inspect and if necessary disassemble, clean and tighten the side terminal battery connections and the connections at the starter. Pay particular attention to the pair of cables connected to the positive terminal on the battery. This stacked connection is somewhat prone to corrosion and poor electrical contact.

If this doesn't help, have a shop test the battery itself and measure the voltage drop and current draw from the starter motor on a hot restart.