Q: I am having trouble finding out what’s wrong with my oil pressure gauge.

My motor home is a 36-foot 1997 model with a Chevy V-8 block. When I drive around 55 mph (I tow a small car), the gauge comes close to maxing out. When I slow down to around 35 mph, however, the gauge’s measure drops to around midway. I know it’s not a good idea to have low oil pressure, but is it bad to have high oil pressure?

I have changed the oil and put in a new oil filter, so I know the vehicle has plenty of oil. Any advice you may give me would be very beneficial.

A: I’m thinking everything may be OK. At cruising speed, the engine is turning at a fairly good clip, making maximum oil pressure. Around 35 mph it’s possible you are still in high gear and the engine is turning considerably slower. This would explain the drop in oil pressure at that time. A gauge reading midway shouldn’t be of concern. Oil pressure varies with engine speed and drops somewhat at higher temperatures.

As a general rule, engines need about 10 pounds of oil pressure for every 1,000 rpm of rotational speed. Around 35 mph you’re probably spinning about 2,000 rpm with about 35-40 pounds per square inch of oil pressure, around mid-gauge. At idle, 700 rpm, it would be nice to see 15-20 psi.

If there is any doubt regarding the true oil pressure, a manual gauge could be temporarily connected in place of either oil pressure sending unit and then the pressure could be checked under a variety of conditions. Correct oil viscosity and condition are also factors. I have found more than a few General Motors oil pressure sending units to be rather flaky and inaccurate. This may explain your borderline high oil pressure readings at cruising speed (not of concern).

Q: What is your opinion on continuing to use tires that are about 10 years old but still have lots of tread and show no signs of weathering at all? I have a classic pick-up that is driven only occasionally and the years pile up a lot faster than the miles.

A: This can be a tough call because tires deteriorate for a number of age-related reasons and it’s impossible to know true condition without cutting them apart in a lab. The consensus among a growing number of vehicle manufacturers is that six years is the maximum age tires can safely reach before needing replacement. The tire industry says up to 10 years. Storage conditions are likely to have an impact. Occasional use, lightly loaded and garaged would be the best scenario. Long stationary periods, a heavier vehicle, exposure to the sun, coastal climate and subsequently driven on hot pavement or at higher speeds would be the most undesirable conditions.

I was one who stretched the envelope towards 10 years but am now thinking six. Two consecutive tire failures on two trips with the car trailer with 10-year-old tires that looked almost new got me thinking differently.