Q: I drive a late-model luxury vehicle that has a lot of mileage. Lately I’ve discovered that the pressure in all my tires is lower than the manufacturer recommends, but I believe the car rides better this way.
So when I took it to my repair shop recently, I asked them to keep the air pressure low. They told me they do not recommend driving the vehicle with the lower pressures. Do they know what they are talking about? I am only asking them to lower the pressures by 4 or 5 PSI (pounds per square inch).
A: Your repair facility is correct in their advice not to run tires at a lower pressure than recommended by the vehicle’s manufacturer. In California, it is mandatory that all repair shops inflate tires to the manufacturer recommendations and also make note of having done this on their invoice. If you give instructions not to allow the shop to check your tire pressures, you must sign for this on the invoice. There are three important reasons for this:
1. Lower tire pressure will have a negative effect on your gas mileage.
2. The tires will generally wear unevenly. The outside edges will probably wear faster than the center of the tire, causing you to need tires sooner.
3. There are safety concerns:
• The surface area that contacts the road is negatively affected. This could cause the vehicle to require a longer distance to stop in an emergency.
• Lower tire pressures will cause them to run hotter at freeway speeds and possibly lead to tire failure.
The tires and vehicle shock struts work together to give the vehicle its ride characteristics. If you have experienced a change in the vehicle ride over its life, one or both of two things might have happened. The tires on the vehicle are not the same or do not match the tires that the vehicle was designed around or the vehicle shock struts are worn past their usable life.
If the latter is true, it means the shock struts are not controlling the movement of the suspension as designed, therefore affecting the feel of the ride.
Under-inflated tires and worn shock struts will both negatively affect your vehicle’s ride, control and gas mileage and they will increase the stopping distance required. Make sure that the correct tires are installed on the vehicle, inflated to the manufacturer’s recommendations and inspect the shock struts. If the mileage exceeds 80,000 or the shock struts are bent, broken or leaking fluid, it is time to install new ones. If the shock struts are worn beyond their ability to perform, the tires will wear unevenly. Do not wait to see uneven tire wear (also called cupping) to install new shock struts. That is way past the point that they should have been replaced.
Another fact to be aware of – bouncing on the front or rear bumper is not a valid test to determine worn shock struts. Be sure a knowledgeable professional at a reputable repair facility performs the inspection.