Tire pressure vs temperature
 

I always check my tires' pressure before a trip, in the morning, before the sun warms them up. I keep them at the max sidewall of 51psi.

A few weeks ago I checked the pressure and noticed that all 4 tires are close to 60psi! Is it possible that if I pumped to 50psi on a cool morning (+5 to +10 C), then it would be 60psi on a warm morning (+15 C)? Or maybe I pumped to 60psi by accident (I often pump our bikes' tires to 65psi)?

Maybe model the air in the tire as the difference in volume between two cylinders and use P.V = m.R.T. Plug in the numbers and see what the difference in pressure works out to be.

I've never personally had this issue (as I'm almost always asleep in the morning lol), however I have been to a few service station air pumps that have faulty air gauges. Not sure how you pump your tires up but it's a good investment to have a good air gauge to check your tires. I've never had good readings from cheap gauges.

The pressure gauge is integrated with the pump I use at home, and this is the only pump I use for the car and bike tires, so this rules out any variations between pumps/gauges.

Our local Firestone Tire dealership has been kind enough to "check" all my pocket air-pressure gauges for me...checked against their *master* pressure regulator which they have checked/measured/calibrated every six months. They have a "T" shaped fitting that chucks onto their line while my gauge(s) connect onto one of the other two fittings...and we compare the numbers.

I'd say this is normal, i find my tyre pressures rise with ambient temperature.

... and what were your results? Were your gauges worth the (little) money you spent on them?

Two (dial gauge) were high at 40 psi...one read 41 psi, other read 42 psi.
One (pencil gauge) was low at 40 psi...it read 38 psi.

I've noted their errors on each gauge and am keeping records to see if those errors are getting worse or staying the same.