How can tire pressure affect an AWD system?

Published 12:00 am Sunday, July 5, 2015

Q: You’ve spent a lot of ink on tire diameter differences caused by unusual tire wear. How about describing the impact of one low-air-pressure tire on the all-wheel-drive system? I run my tires at 35 pounds per square inch except for one wheel with a leaky alloy rim. Sometimes it is down to around 25 psi before I get to it. How much is that changing the effective wheel diameter (and affecting my AWD mechanicals)? This is a situation that many cars with alloy wheels experience in cold winters.

A: I’d never really thought about this scenario before, so let’s explore it. Intuitively, it seems logical that a significant decrease in air pressure would reduce the rolling diameter of a tire. That’s confirmed by the fact that several tire pressure monitoring systems monitor the distance between the axle centerline and the pavement — a significant drop in pressure would give a shorter reading on the sensor, which is interpreted as low tire pressure.

But the question here is: How much will the tire’s rolling diameter change as its pressure drops?

Being the inquisitive type, I went out to the garage to use our Tahoe for an experiment. It has 265/65R18 tires worn about 60 percent — very close to replacement time.

I used a plumb line dropped from the top of the tire, through the wheel’s centerline to the garage floor. I marked the tire and the floor at the plumb bob’s point. With the tire inflated to its normal 35 psi, I rolled the vehicle backward exactly one tire revolution and marked the floor at that spot. The distance between marks on the floor was 97.3 inches. I then dropped the tire pressure in that tire to 25 psi and rolled the vehicle forward exactly one tire revolution and marked that spot. The difference between the two marks was just under 1⁄4 of an inch.

Using the equation for diameter — circumference divided by pi — the 35 psi tire was 30.97 inches in diameter while the same tire at 25 psi was 30.90 inches in diameter. The difference was .070 inches — just under a tenth of an inch or just about 1.0 percent. This would not be significant enough to affect a 4WD/AWD vehicle.

The numbers might change a bit for a significantly smaller tire, but probably not much. If tire pressure drops into the danger zone — below 20 psi —you’ve got bigger worries than tire diameter.

— Paul Brand is an automotive troubleshooter, driving instructor and former race-car driver. Email him at paulbrandstartribune.com.