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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 AWD system? I run my tires at 35 psi 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)? Now, that’s a situation that many cars with alloy wheels experience in our winters.

A: Fascinating question! 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 .220” — just under ¼ of an inch.

Using the equation for diameter — circumference divided by pi — the 35 psi tire was 30.97” in diameter while the same tire at 25 psi was 30.90” in diameter. The difference was .070” — 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.

Fun question, thanks!

Q: I have a 2005 Ford Focus with the 2.3-liter engine. A couple of weeks ago the alternator went out and I replaced it with a rebuilt one. Since then the warning light comes on randomly. The new alternator and the battery are good; I have checked them several times. I have even been checking them, watched the dash light come on, and the needle never twitched on my meter. The dealer wants $80 to $200 to diagnose, and $350 to $650 for a dash module. That sounds like $500 to $1,000 to get the light working properly — not my plan. Besides ignoring the light, what options do I have?

A: Once again, apply the KISS principle. Check all the alternator and battery connections carefully. You can’t go wrong in disassembling, cleaning and reconnecting all these. Check the alternator drive belt for wear, contamination and signs of slippage.

Many of today’s vehicles have “smart” charging systems that utilize the PCM to control and monitor battery charging. For example, no recharging is allowed until the engine has started. The battery is recharged at a higher rate when the battery is cold and at a reduced rate at high engine outputs — all designed to maximize charging efficiency. I’d also suggest double-checking that the replacement alternator is the correct application for this specific vehicle.

If no answers yet, continue the KISS process by having the PCM checked for fault codes at a parts store that offers free scanning. This might be the key to pinpointing the issue.