Does anyone know how distance is measured by a GPS unit ??
What I want to know is it the distance as the crow flies or does it measure every turn you make ??
The reason I want to know is because I want to get another measurement of fuel mileage on the oddy's.

Depends on the settings and device you are using. You can have one that measures your distance from a set point, or you can have one measure the distance on the path you have traveled. I think most default to the distance of total travel, but again, its based on the unit you have and the settings.

Take the GPS in question for a ride with you in your automobile compare distances with the odometer...

I like that idea .
Thanks for the help guys.

most GPS will have a refresh rate, not sure the older Garmin's were 1 sec, so if you made a sharp hairpin turn you may loose a little of the distance, otherwise I think they are pretty accurate.
they are going to measure your track.