You’re totally right — some covers just add bulk and end up hurting more than helping. I switched to a sleek hard folding one from
Undercover and saw a noticeable improvement on highway trips. I wouldn’t call it dramatic, but over long distances I was saving maybe 1–1.5 MPG, which adds up. The cover sits nearly flush, so it doesn’t catch wind, and the truck just feels smoother when driving. Plus it’s got solid seals all around and a clean finish. Didn’t expect a tonneau cover to do much for fuel at first, but this one changed my mind.