"I've sometimes wished we could all have meters in our car, rather like in taxis, which would show us how much it is costing at the speed we are travelling.   Saving money isn't the highest motive, but it might be quite effective. And one thing no one had mentioned is that the faster we drive and the more fuel we burn, the more damage we do to the environment."

Most gasoline auto engines provide the best fuel efficiency at approximately 2000-2500 r.p.m. This translates into about 100kmph. It depends on whether your in a 4, 6 or 8 cylinder vehicle. So driving slow is not going to work all that well.