Q I bought my 2004 Suburban new, and it now has 100,000 miles on it. There is a howling in the front axle which I have been told is the front differential assembly. One garage wishes to replace this assembly for about $2,000. Another garage says the problem is that my one front tire is new and has a larger circumference than the rear tire matching it. He says all four tires have to be exactly the same circumference before the differential assembly can be replaced. This sounds strange to me; what say you?

A The second garage is correct. With any four-wheel-drive or all-wheel-drive vehicle, it is critical for all four tires to be the same size and diameter. With one tire significantly larger or smaller, the differential on that end of the vehicle will think the vehicle is turning -- one wheel rotating faster or slower than the other -- while the rest of the drivetrain thinks the vehicle is traveling straight with all wheels rotating at the same speed. This can put enormous mechanical stress on driveline components and lead to premature failure of expensive components.

Start by inflating all tires to the same air pressure -- 35 pounds per square inch works well -- then measure each tire. Put a chalk mark on the tire and the pavement at the bottom center of the tread contact patch, then roll the vehicle forward exactly one tire revolution and mark the pavement again. Measure the distance between each chalk mark with a tape measure. All four tires should be very close -- a difference of a half-inch or less in circumference.

You didn't mention whether your vehicle is equipped with full-time AWD or automatic 4WD. If the noise is still there once all the tires are the same diameter and your vehicle has automatic 4WD, see if the noise goes away when locked into 2WD. If so, the front differential warrants a closer inspection.

Q I have a '99 Chevy Metro that has exhibited a chronic problem since Day One. After being driven for approximately 30 minutes, it seems to lose brake boost. The pedal goes rock-hard. The brakes continue to function, but considerably more force is required on the pedal to slow the car. The weird part of this malady is, almost always, the next time the brakes are applied they function normally. I realize that it's a safety issue -- any ideas for a possible remedy?

A You're describing a loss of vacuum power assist available to the brake master cylinder. It's possible that a lack of engine vacuum due to restrictions in the air intake or exhaust systems or some other engine performance problem is the cause. Otherwise, look for a vacuum leak in the vacuum booster or its check valve. To check this, with the engine fully warmed up and idling, shut off the heating, ventilation and air conditioning system completely, then shut off the engine. After the engine stops, there should be enough residual vacuum in the booster to allow several brake applications before the pedal gets "rock hard."

Q I have a 2003 Anniversary Edition Corvette with less than 10,000 miles. Is it time for a coolant and transmission flush?

A I think so. Based on age, both services are already overdue. And since this is probably your dream-of-a-lifetime car, why worry? Get 'er done.