Microlighters

On straight and level, with bar neutral, my ASI reads 42 mph. Plane flying alongside registers 50 mph. Always thought that my ASI was wrong. Is there any way to calibrate / adjust it?
It is a venturi type. (no pitot tube)

Here is a spreadsheet. Using a GPS on a zero wind day take off and fly a triangle. Make sure you fly at equal speeds on all the legs. Try a fast triangle, a med and a slow one.

Import your tracks from the GPS or take down the Ground Speed and the heading in deg for each track at each speed.

Drop the results in the spreadsheet and it will tell you how far out your ASI is.

I don't know how to fix the venturi type, nor the normal one for that matter but at least you will have an accurate indication.

Aviation instruments at the Aviation Business Centre might be able to repair it for you or, alternatively have a card that says, 42mph ASI =50mph etc on the dash