Thanks dcb, I did not think of it like that. If I read the gal/hr and divide it into mph, I'll get mpg! If I can do the math that fast.
I am thinking about mounting some sort of optical sensor to read the driveshaft rotations. I might try a small variation on this:
http://www.punkjob.com/OpticalIFs.pdf Not ideal, but I think with the right positioning it might work. If I can shoot the light beam to get interrupted by the yoke turning right behind the transmission...