One might assume that a smooth surface is the most aerodynamic, which is why keeping cargo off the roof rack gives you better gas mileage. However, with an entire vehicle, when considered as a system, aerodynamics becomes a little more complicated. Some automakers, such as Toyota, have added additional panels under the body, both smooth and dimpled, to decrease drag under the vehicle, increasing fuel economy.
Yokohama Rubber Company Limited has developed a new tire design that aims to reduce overall vehicle drag by increasing the drag of the tire itself. Adding fins to the inboard sidewall might seem like it would increase resistance, which is exactly what it does, but the overall effect on a vehicle’s aerodynamics is negligible.
On the other hand, perhaps a little bit of drag is exactly what is needed in the wheel well, where the new tire design would do its magic. During normal vehicle operation, at highway speeds specifically, air flowing in and around the tire and wheel is flowing in random directions. In some cases the air flow actually contributes to vehicle drag, and fuel economy suffers.
Yokohama’s testing has found that their new radical tire design creates a spiral flow of air in the wheel well, which, when combined with the entire vehicle system, reduces overall vehicle drag. The air is propelled from the wheel wells in such a way as to add a slight push forward. As automakers try to squeeze even more miles per gallon from their vehicles, I can imagine that fin technology combined with low-rolling-resistance tire technology might be a great step forward.