Q. I recently purchased a 2006 Buick with 45,000 miles. I have put only 5,000 miles on the car. When I reported a vibration to the local Goodyear Tire store where I just bought four new tires, I was told I needed front and back shocks and a new transmission mount.  My mechanic took the car for a drive and disagreed with Goodyear’s recommendation.  He seems to think it is one of the front wheel bearings, not the shocks, and not to worry about it for a while because it isn’t serious. I drove the car on the highway, and it tends to skip and vibrate on bumps at high speeds. The mechanic said this is a characteristic of the Buick air cushion ride. I looked at the rear shocks and fluid is leaking. Should I replace the shocks?

A. The vibration you describe is characteristic of worn shocks. The shock absorber’s job is to keep the tires in contact with the road over bumps. Oil leaking from a shock is also a sign of failure. A worn wheel bearing will typically make some noise and will generally only cause a vibration when it becomes badly worn. Although the air suspension system adds a “floaty” ride, it doesn’t skip or vibrate over bumps.

Q. My question is about checking tire pressure. All the standard recommendations I see about my car and in the tire shops are given in PSIs. However, the majority of air tanks I've used to refill my tires have built-in gauges that give readings on the order of
1 through 4, not the 20 through 40 or so that I would expect for PSI. Do you know the units that these gauges are using?

A. I have seen gauges read in kilopascals and atmospheres, as well as pounds per square inch. Regardless of what kind of gauge is on the tank or air station, purchase a good quality tire gauge. Most tire gauges will read in the familiar (PSI) pounds per square inch. Check the tire pressure once per month when the tires are cold and inflate them to the recommendation on the vehicle door placard.

Q. I had a Subaru and liked the car until the head gaskets started to leak. I’m considering the new Forester. What do you think of it and have the engine problems been fixed?

A. The new Forester is a very nice vehicle. Over the years, it has grown in size and the overall fit and finish have improved. The engine design is now a PZEZ low emissions engine that returns better fuel economy than earlier models. I recently drove a Forester and routinely saw mileage in the high twenties. As far as I know, the head gasket problem that was common a few years ago has been solved.

Q. My 2002 Chrysler Caravan with an automatic transmission surges after it shifts above 40 miles per hour. I had the transmission fluid changed, and it’s at proper level. It seems to shift well around town at slower speeds. Do you have any ideas? I don’t want to be “taken” at a transmission shop for something that is not the problem.

A. The problem could be with the computer in this model. Some of these Caravans had a problem that would cause a surge at 40-50 miles per hour that was most noticeable going downhill. The repair required replacing the computer and updating its software.

Q. I have a 2009 Corolla that calls for 5W-20 or 0W-20 oil. While changing the oil last week, I mistakenly mixed two quarts of 5W-30 Toyota branded oil with 2.4 quarts of 5W20 Toyota branded oil. I was wondering if I need to drain and re-fill with 5W20 or if this is safe until the next scheduled oil change.

A. Toyota is very specific about using 5W-20 or 0W-20 oil in this model. Mixing the two grades together most likely resulted in the heavier than specified oil. Twenty years ago, I would tell you not to worry about this mix of oil, especially with warmer weather approaching. Today, with such tight engine tolerances and the possibility of different additives in the two weights of oil, I would be tempted to refill the engine with the oil recommended by the manufacturer.