They already advertise MPG/ LKM. Fuel varies so much in price that fuel price variation would mask cost differentials between cars. You could start combining

They already advertise MPG/ LKM. Fuel varies so much in price that fuel price variation would mask cost differentials between cars. You could start combining some synthetic averaged fuel price with MPG to arrive at a cost. But is it a cost that anyone's likely to achieve, or know how it relates to their own circumstances. At least mileage figures however flawed are a known quantity, I know how it relates to my driving, and I know whether fuel prices are currently low or high.