try out that app on your phone called Fuelly. it's easy to keep track and get a average over several fuel-ups. also keeps track of receipts and money spent.
biggest things that change mileage are wind direction along with going up hill and down hill plus elevations. if you want to get picky, don't fill it to the brim, I always do the one or two clicks when the nozzle shuts off and call it. print the receipt from the pump and enter all of the info into the app.
there are a couple of guys here in the forum that use the app and have the results in their signature like me!