Effectiveness of a rear air deflector

I was passed by a later model 4Runner this morning. Toyota added a rear air deflector to the T4R after my model was made (without one).

I’ve always assumed that the deflector’s purpose was to direct air into the partial vacuum behind the vehicle and reduce the suck factor.

Is this what they’re for? If so, how effective is it? Would it add any MPG to my admittedly poor gas mileage vehicle?

It’s purely a “someone told me this” but I had an SUV with a deflector at the back and was told that it deflected air down the back window to help keep it reasonably clean. Otherwise, dust tended to accumulate on it very quickly due to the fairly static (and dirty) air mass at the very back of the vehicle.

This is the same purpose given to me by a dealer.

I believe Turek is correct. Ford used to have these on their big station wagons in the 1970s

Back window deflectors were offered as options on a lot of station wagons in the early 1960s and by the end of the decade were standard features on many (most?) models. There were even fancy versions that were built in to the body–some on the roof, some on the sides where car-top carriers would not interfere with their ability to keep dust off the windows. Our first three wagons did not have them and when we first got a car that did have them I was pretty impressed with the way that they actually worked (since so few gadgets actually seem to do what they are intended).

I have always been mildly surprised that they were not employed on mini-vans or the larger SUVs when the behemoths were first introduced. On the other hand, there are a lot fewer gravel roads than there used to be, so it may simply be a matter that few drivers felt they were worth the bother.