Air pressure gauge
 

I'm getting my Boxster ready to store and increased the tire pressure to 58 psi (as recommended). I'm used a Shop Force household compressor (about $40).
After increasing the tire pressure I checked it with a hand-held tire gauge and it measured about 50 psi. It has a rotary gauge (Accu-gage) and cost $6.

So, the gauge on the compressor says 58 but the handheld says 50. I guess I don't trust either measurement.

Where can I get a gauge that I can trust?

Longacre makes a good one. As does Porsche($20)
That tire pressure sounds high.

I use a digital gage, from any retail auto parts store, very accurate, easy to use, costs about $10. I think it's called a Monkey Grip Gage.

Perfectlap, yes the pressure is high. But. that's the recommended pressure stated by Porchse in the owners manual for long-term storage. He's doing the right thing here.