I read all of the time about improvements that have been seen or predicted by the addition of tonneau covers. I guess in a perfect situation, they are probably true, to an extent. However, in my experiences with a toyota, chebbie and, currently, my PSD CC, I saw in the real world over extended periods of time absolutely no change in the fuel consumption. If there was any, it must have been so minor as to not be able to calculate it. But in defense of the tonneau covers, I never purchased any of them with the expectation of mileage improvements - they were purely for their practicality, protection and appearance. I do like them!