I've got a rotating bezel driven by a float in the Auster, ie completely mechanical. It's the original 1946 gauage as far as I know. On the ground it underreads by 3 gallons and in straight & level flight it underreads by 2 gallons. I've verified this by putting a known quantity of fuel in it. At least it errs on the safe side! It's nearly as accurate as the wire on a stick that is fitted to the J3 / L4 Cub.