TPMS Fact I didn't know
 

Took the Q5 in for camera, and while there inquired about why the TPMS did not indicate when wife got flat (that's a different post!). As I knew, the system works by noting the difference in rotation between the tires. A difference being interpreted as different pressure. What I didn't know was that (according to the dealer) it takes about 3 miles of driving in this condition before it will alarm.
Makes sense in a way, as you wouldn't want minor changes due to temp setting off the signal. My Acura drives me crazy every year when the temp drops below freezing.
By about 3 miles, my wife had shredded the tire and was parked, so no alarm. Anyone else heard this story?

I had not heard that but it makes sense.

The trade off is that the old system had sensors that needed to be used and meant another US$100+ per wheel if you used a set of snow tires and wheels. Also these sensors were ruined if you used the tire sealant. They also have a internal battery that requires you to replace the sensor when it dies.

Wow - sorry...
I've not heard of it happening elsewhere either (thank goodness !)

would think there would be other 'indications' if you had a tire go way down quickly - like handling issues, 'squishy' feeling...

Like my Dad used to say
"How did we do it before(fill in the blanks)came along?!?"

we are SO spoiled aren't we :) :)

Yeh, she said it did feel a little "funny" and the sun roof rattled. Makes me think if she hadn't stopped at the grocery store she would have driven it until the rim wore out. We have had subsequent discussions!

lol

Unfortunately my 09 has the in wheel tire pressure sensors so I had to buy 4 for my winter wheels at $60 ea. However my previous BMW X3 had the system based on the abs wheel sensors and measuring the difference in rolling diameter but it told me a tire was low within a few hundred feet. Temp variations would't be an issue because all the tires would vary equally. The system should only measure the difference between tires if one is low.

Not an expert, but think the variation is not between wheels, but between what the MMI is set at when initialized and if it changes later...

my thought was that when you selected the "Tire Pressure is right" option, it recorded the revolutions per mile and then if the revolutions varied by more than some factor it warned you....

and would think it's for each tire....

is that not correct ?

As a note my neighbor just bought a new 3 series wagon and they are back to using sensors in the wheels.

The Audi system currently being used is actually ridiculously fancy and monitors "abnormal vibration" as well as rotational inertia. It's supposed to be able to detect slow leaks from all tires at once, or a sudden deflation of one tire.... Supposedly within the NHTSA's requirements on sensored TPMS.

Also, though not exactly related to the op's original question, the TPMS only says something like 'tire pressure is low, check all four tires'. It does not tell you which tire is low. If I didn't have a tire pressure gauge in my glove box a few weeks back I would not have known which tire was low (aside from eye-balling it which isn't always accurate)... Not sure why it doesn't just tell you which tire.