Q: I've always read that it's cheaper to buy cars three to five years old that have substantially depreciated. With used cars now holding such high prices, does new or used make more sense?

—George Rau, Grand Junction, Colo.

A: New-car sales fell during and after the 2009 recession, so there have been fewer late-model used cars on the market since then. The limited supply pushed prices up to where new cars were better values in certain cases. Still, used cars are usually less expensive overall, as long as the buyer does a lot of homework, including a thorough inspection by a good mechanic. Also, some analysts predict an increase in cars coming off leases this year will drive prices down.

Q: For a few extra dollars, the automatic wash sequence at the local carwash adds some liquid wax to the mix. Given how much elbow grease is needed to hand-wax a car, it seems unlikely that a few seconds of a spray of wax would do much good. Is this treatment useful?

—Peter Behuniak, Glastonbury, Conn.

A: At best, a carwash wax treatment serves as a mild touch-up between hand waxings. But often it is a disappointing and expensive shortcut. I like picking my own wax and having more control over where it goes. There really is no substitute for waxing a car by hand.

Q: When I took my 2012 Honda CR-V to the dealer for an oil and filter change, I was told that "they" recommended replacing the fluid in the rear differential. I was surprised that this one service item was $110. Was it necessary?

—John Reidenouer, Waynesboro, Va.

A: Last time I checked, the regular interval for that service was 90,000 miles. Some drivers change it sooner, because CR-Vs have reportedly had problems with fragile differentials. You can ask for a change when the mileage is right or when you get a signal from the vehicle's electronic maintenance reminder.

Q: The plastic headlight covers of my 2003 Mercedes-Benz E320 have now accumulated a fog-like haze. Is there an easy way to remove this haze, and what commercially available cleaner might you recommend to do so?

—Jerry Burke, Washington, D.C.

A: Automotive plastic polish—the type used to revitalize foggy plastic rear windows on convertibles—will do the job. There are "headlight restoration" kits with detailed instructions. I have found the mild abrasive in toothpaste works well in a pinch.

—Email mecar@wsj.com