Puzzler Question: Monitoring Tire Pressure with a Snap-On Scanner

I would surprise me if a Snap_On scanner couldn’t predict the temperature in Beijing on Super Bowl Sunday.

Car Talk’s Air Pressure question was first aired on Feb. 15, 2003. Puzzler Answer: Snap_On Scanner Scenario. At the time, their answer was correct. The government was about to institute a requirement that cars be able to detect low tire pressure. As the relative tire rpms was already measured in the ABS system, it would be simple (and cheap) to use these data to detect low tire pressure in a single wheel. (Doesn’t help if all four tires have low pressue, however.) The method is known as the Indirect Method for measuring tire pressure; I think Audis and some VWs had them back in the day.

Nowadays, most tire pressure sensors use what is known as the Direct Method which relies upon rf transmitters mounted either inside the tires on on the air valve nipple stem.

For more information, check out Tire Rack: Tire Measuring Monitoring Systems.

The ABS measures the Richard Petty Motorsports? What is “rpms” in this context?

Ah, I see now. What a terrible way to measure tire pressure.


I'd prefer that to "littlemouse." Hope you have a healthy and happy 2012.

I see what you did there. That was the kiss-slap/slap-kiss. The passive aggressive love-hate hug.

I’m disappointed but not surprised.

If it was 03 maybe it made some sense but in 2012 the computer has that information so if you are scanning the computer, why wouldn’t it contain the air pressure? On Star gives me mothly reports including what my air pressure is per wheel. Its also on board. Seemed like a lame question.

I must be missing something. When the pressure in the tire drops, it doesn’t change the circumference of the tire, does it? That is, the length of the tread remains the same. So if there is no slippage between the wheel and the bead of the tire, and no slippage between the tread and the pavement, wouldn’t the wheel move the same distance for every revolution as it does when fully inflated? Or to put it another way, wouldn’t the necessary rpm of the wheel, for a given speed, remain the same? (Imagine a tank tread - would the tank go further per each revolution of the tread it the tread were arranged in a circle, instead of a flattened shape?) Or, does slippage between the tread and pavement necessarily occur when the pressure drops a certain amount?

A pressure drop does decrease the circumference slightly.

I agree with texases. A decrease in circumference would be the only thing that would affect the RPM of the wheel. As the tire pressure decreases, the circumference gets a bit smaller.