[Fat Charlie]

Focus-Ak- So you eyeball the car's odometer against the measured mile. Yes, waiting for the "click" can give you somewhat arbitrary start and end points, but you can line everything up and in the worst case you can measure the offset at the start and end points.

Then you calibrate the aftermarket gauge to the odometer over the course of a few tankfuls. Being a couple tenths off after a few hundred miles is accurate enough in my book. Keep checking it, though- I bump the numbers every several tanks when I've been able to use the same pump a few times in a row.