Ok, married the meter to a MPGuino for a road test, piece of balsa/hot glue, my fave
OBSERVATIONS:
1. The refresh rate of 1/2 second makes the meter seem very un-analog. While having a refresh rate works well with an LCD (gives you a chance to read it), the analog gauge should probably be updated with every injector pulse and the average 1/2 second of speed.
2. It needs a backlight.
3. It was a bit bouncy with the road, one of those aftermarket tachs might make a better in-car meter for this sort of thing (might even have a backlight).
So room for improvement certainly, and an instant mpg without a trip mpg is of dubious value unless you are a complete hiway star. But it is certainly a doable thing with very little hardware.