Not to be an ass, but why? Most new cars have them, hell my buddies cobalt has one, doesn't mean its a nice car. I think its lame, kinda like the compass they put in a lot of GM cars. Most of these "MPG Gauges" are inaccurate at best. If you care that much then go for it, I don't wanna stop ya.
But ya know if you know the number of miles you've driven, and divide it by the amount of fuel used, it gives you your MPG, without the extra work weight and power drain of a guage thats pointless at the end of the day.
Sorry if I come off as a prick, but I wouldn't do it.
Good luck with it if you try to make one.