How do I measure the benefits?

There are many ways that you can measure the benefits. A lot of modern cars have on board computers that give you the mpg, but these are not always accurate. The best way is to reset your odometer when you fill your tank, use the car until almost empty, then divide number of miles travelled by number of gallons used to fill your tank back to full. (there are 4.55 litres of fuel in a gallon).