Q.The self-sealing tires on my 2005 Chrysler 300C all-wheel-drive model with a 340-horsepower Hemi V-8 aren't lasting as long as I expected, with the first set being replaced at 29,000 miles. -- H.M., Chicago

A. Premature wear with original-equipment tires for that model isn't unusual. Goodyear's Assurance Comfort Tread tires are suggested by experts at the Tire Rack (888-371-8473), the country's largest independent tire tester. They're not self-sealing tires, which really aren't necessary.

Q. I've started to use regular-grade gas in my car, although its manufacturer says it calls for premium, which has become too expensive. The engine has slight "knocking" and doesn't perform as well but drives good enough to satisfy me. I wonder, though, if I'm damaging the engine. -- L.A., West Chicago

A. You didn't say what type of car you drive or if its owner's manual says regular-grade gas is OK during moderate driving -- but I don't like the engine knock and performance loss you're experiencing. I'd return to premium fuel to avoid possibly costly engine damage. Most cars increasingly only need regular-grade fuel, which is partly why premium nationally accounted for only 9.4 percent of gasoline sales last year, compared with 16.6 percent 10 years ago, according to the Energy Department.

Q. This may sound crazy, considering high fuel prices, but is now a good time to buy a used, late-model SUV? I need the utility provided by such a vehicle and wouldn't drive it very much because I have cars that get good fuel economy. -- D.W., Western Springs

A. You couldn't pick a better time to buy a used SUV because it's a buyer's market. The Kelly Blue Book says prices of used SUVs have fallen more than $2,500 and some by more than $3,500. Moreover, prices may go lower if fuel costs even stay at current levels. Many got a three-year lease in 2005, when gas cost less, and are due to turn in their SUVs, which finance companies will want to quickly unload.

Q. With all the national flooding in the Midwest, how can I prevent buying a used car with water damage? -- C.K., West Chicago

A. Few things are worse than water damage, especially with all the electronics in modern cars.

The National Insurance Crime Bureau is creating a free public database of vehicles declared a total loss, "or totaled," by insurers, allowing individuals to enter vehicle identification numbers, or VINS, to see if a vehicle they want to buy has a hidden history.

The online service begins at the end of this month and can be found at www.nicb.org. Look for a link to the organization's VINcheck system.

The bureau says nearly 12 million vehicles have been "totaled" by the insurance industry, most of them after collisions, but also after fires and floods. Industry experts say about 30 percent of these vehicles are "recycled" into the used car market.

Q. Why do automakers use numerically high axle ratios, which allow fast acceleration but lower fuel economy? Isn't it better to get 30 mpg than the fastest 0-60 mph times? -- B.L., Burr Ridge

A. Most automakers use a "compromise" final-drive axle ratio for the best blend of performance and fuel economy. Yes, a numerically lower ratio would increase fuel economy because it would let an engine work slower and thus use less fuel. But make the ratio too numerically low, and autos would be too slow to safely merge and pass.

Car question? Send it to Dan Jedlicka, Chicago Sun-Times, 350 N. Orleans, Chicago 60654, or e-mail djedlicka@suntimes.com. Include name and hometown, and be sure to include a self-addressed, stamped envelope to ensure a personal reply.