WhoDat,
I have an 01 therefore I do not have the on board MPG cal or whatever it's called that the newer models have. I use a method I got by googling it. I fill up at a certain gas station paying attention to where my gas needle is, drive until it's time to fill up again. I try to fill up at the same gas station and with the needle being at the same spot. I write down the miles i have driven, then divide that by the number of gallons it took to fill it up and this gives me my MPG.
I don't know of an easier way than this so if someone else does please chime in. I keep thinking there has to be a better way of figuring it out than this method but haven't found one.