Right, what VehiGAZ said - and most importantly, understand what iamjacksadrenalgland mentioned about having larger than stock tires - even if you are doing perfect calculations, your data is inaccurate because larger tires throw off your speedo/odo, so it may appear that you are getting much worse mileage than you actually are.

I've got 33" tires on 16" wheels, and I estimate that my speedo/odo are off by about 10%. Adjusting for that difference, I probably get 12-15 MPG on average.