Leakage rated vary. My cars leak air between one-half and one PSI every two or three weeks. Driving the cars seems to increase the leakage rates. Checking your tires only ever two months is not enough. It's particularly not enough when the outside temperatures are steadily decreasing, e.g. early-December to early-February in Canuckastan
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From the Ideal Gas Law, tire pressures go up or down about one PSI for every ten degrees F.
You could also have an embedded nail or screw. It they're headless, they're hard to find visually.
Before FTM and TPMS's, my "flat tire detection system" was to adjust the pressures precisely with a dial pressure gauge, so that the pressures of the two tires on the same axle are identical. I'd do this early in the morning, when the car is completely cooled off, and before sunlight hits the tires (heating them up and changing the pressures). If the pressures on an axle are different in two or three weeks, I'd go looking for a nail or screw and always find one.
I even out tire wear by adjusting the pressures. I usually do it in two PSI and rotation stint (5k to 7k miles) increments. At these increments, there are measurable differences in wear patterns. So, running tires at nine PSI below where they should be will quickly have a significant, negative impact on the wear patterns.
I used to get paid every other Friday. My routine was to check all the cars' tire pressures the Saturday morning after pay day. It's also a good idea to check tire pressures before and after a day on the road. I only get paid once a month now, and I drive less. So, I now sometimes go three weeks in between checking pressures.
I'm taking it that that speed of 120 is 120 km/hr, which is 75 MPH is 'murican units.
The key to long tire life is "A.I.R.:" Alignment, Inflation, Rotation. Doing this, I'll get 45k miles out of my Conti' DW's (tread wear rating of 340) on my Cobalt, almost 40k miles out of my Michelin PSS's (tread war rating of 300) on my 535i, and between 55k and 60k miles out of the Bridgestone Dueler's (tread wear rating of 500) on Frau Putzer's X3.
My top five tire lifespans have been: 79k, 74, 70k, 70k, and 68k miles. I generally keep cars in integer increments (1, 2, 3) of sets of tires, leaving a few thousand miles on the last set of tires for the next owner.
I have to measure and top off my tires in the driveway, because my garage is too cramped. I do this in the early mornings, when all the old geezers in my 'hood are out getting in their "10,000 steps." My garage door is 31' from the public sidewalk (driveway is 1.9 cars long... WTF?). I top the tires off with a good bicycle hand pump. The geezers are always asking me "Do you have a flat tire?" This is one of the problems that will be solved with our next house, with a garage large enough that I can scoot in between the cars on my mechanic's stool inside my garage.