Are you kidding thats stupid easy to make I have been using one for years. just make it automatically subtract the old mileage from the new mileage then divide by the amount of gallons. I will upload mine and you can mess with it if you want but theres not much to it really.
http://plaza.ufl.edu/wjh/temp/mileage.xls
http://plaza.ufl.edu/wjh/temp/mileage.xls