Q: Is it true that you can't replace just one bad tire on an AWD car but have to buy all four new tires?
— Cameron Wiley, Portland, Ore.
A: When our Subaru Outback lost a tire to a pothole in late 2007 I decided to get just one tire because the car was fairly new and had gone less than 4,000 miles. I wouldn't have tried this if the tires had, say, 20,000 miles on them because one tire would clearly have a larger diameter than the other three. Car makers tell me this can affect all-wheel-drive and stability-control systems that may sense a difference in wheel speeds and mistake it for a loss of traction. So what I did is generally not recommended. However, more than three years and tens of thousands of miles later, the car is fine. You have to judge based on how much wear the tires have.
Q: Blue lights, white lights and even a pinkish headlight beam at me when driving at night. The blue are most disturbing. Can you explain the reasoning behind such headlights?
— Randa Pearson, Long Beach, Calif.
A: About 20 years ago when bright xenon headlights were new and before car makers learned to adjust them properly, they often cast a bluish light that oncoming drivers found annoying. But technology has advanced to the point where the beams are so well-focused that they are less bothersome. The cases you mention may be poorly adjusted xenon lights or poorly adjusted aftermarket lights with colored lenses to make them appear "high end."
Q: I've become interested in diesel vehicles again due their fuel economy, power, and lower maintenance requirements. Vintage models revved low, had terrible emissions, but could last up to a million miles. Newer models burn cleaner, rev higher, and have great torque. What's the realistic longevity of newer diesel motors?
— Alex McTavish, Chimacum, Wash.
A: The newer diesel engines benefit from advances in machining and metallurgy that should help them last even longer than the seemingly bullet-proof engines you recall from 25 or 30 years ago. However, the high-pressure fuel systems and complicated electronic controls that give newer diesels such amazing performance are likely to be fragile compared with the older motors' simpler systems. So it's a trade-off. The newer engines themselves (the pistons, rods and shafts) will endure, but their ignition, fuel and emissions systems are likely to need frequent and expensive attention.
Q: I wondered if anything has been written about the predicted performance of the new Chevy Volt and any other battery dependent new models in so much winter weather. Are these new battery and electric dependent cars going to be "lemons" in locales with lots of winter weather?
— Chris J. Krisinger, Burke Va.
A: An electric car's battery power doesn't last as long in cold weather largely because batteries operate best when at their optimum temperature and quickly lose their endurance in winter temperatures. But electric cars also use their batteries to heat the cabin, operate other accessories and, in many cases, to heat the battery so it can work more efficiently.
The Chevy Volt I tested this winter lost about 45% of its range (from 40 miles to 22 or so) when the outside temperature was in the 20s. Electric cars typically have systems that warm the battery during charging so at least you start off at the right temperature. Still, the power lost in keeping it warm is noticeable. As a result, electrics aren't likely to catch on in the Snow Belt.