For the hybrid car drivers that take their miles-per-gallon monitoring very seriously, there´s an online tool on hybridcar.com called the Gas Mileage Impact Calculator.
The calculator at hybridcars.com
allows users to compare hybrid and non-hybrid vehicles side-by side. Users can see how much each vehicle will likely spend in a year on gas, as well as the amount of carbon dioxide, carbon monoxide, sulphur oxide, nitrogen oxide, hydrocarbons and particulate matter the two vehicles are likely to emit.
Other features allow users to make the same comparisons based on an anticipated lifetime ownership of ten years and see how your driving style (e.g. speeding, coasting to a stop, jack-rabbit starts, etc.) will change the gas usage, gas costs, and emissions.
Serious users can track specific mpg, costs, usage, and emissions over time and see the results on a bar graph. If one of the tank-by-tank inputs is skipped, the calculator will average it over time.
Go here to use the calculator
and click Begin.