As I restore my Thruster, I have wondered about the instruments that came with it. I decided to do a couple of tests.
First, the ASI. hooked it up to a pitot tube and fitted same to my car. As the instrument reads in mph, it was easy to do a conversion for set speeds. Luckily I have a nice long motorway nearby.
Whilst the gauge worked, my test wasn't conclusive due to the pitot being too near to the car and was effected by pressure waves from the slipstream. I 'm fairly sure it will be ok in still air.
As for my Altitude gauge, I simply looked up the height of my home on Street View in Google Earth (406feet), set the gauge and took a drive in to the hills. Mount Lofty, the highest point overlooking Adelaide is 2,385feet and the gauge read true at it's summit.