The part that I roughly understand: 1) Drag equation, drag increases by square of velocity, thus if it was only factor to be taken in to account then the lower speed the better. 2) In case of cargo ships there is a policy of decreasing speed to reduce fuel usage, called "slow steaming". It became really trendy during recent crisis as there was excess transport capacity and very price conscious clients. 3) Actually for cars the fuel efficiency picture shows also problem of too low speed: OK, so my question: -why low speed reduce so drastically fuel economy of contemporary designs? -if one wanted to design a car that works perfectly fine (or even better) with low speed, how should be the engine (?) redesigned?