How much better mileage do automobiles get at 55 MPH compared to 70 MPH?

Automobiles generally achieve better fuel efficiency at lower speeds due to several aerodynamic and mechanical factors. When traveling at 55 MPH, cars experience less air resistance compared to when they are driven at 70 MPH. The increase in speed leads to a significant rise in aerodynamic drag, which can decrease fuel economy.

Research indicates that fuel efficiency can drop by as much as 20% when speeding from 55 MPH to 70 MPH. This is because a vehicle's engine has to work harder to overcome the greater drag at higher speeds, thereby consuming more fuel. Consequently, maintaining a speed of 55 MPH allows for more optimal performance of the engine, leading to improved fuel mileage.

The other options, while alluding to improved mileage benefits, do not fully capture the extent of the improvement seen at these speeds, making the 20% figure the most accurate and reasoning-based choice regarding fuel efficiency differences due to speed variations.