[HardcoreZ28]

So I'm thinking about building a tonneau cover for my truck. I found some good plans online the other day. My main reason would be to help with gas mileage. Does anyone know roughly how much of an improvement in mileage a cover usually makes? I've read 10% but that's from the manufacturers. Although if I could pick up even 1mpg extra it would be well worth it. Also I'd probably make it a little trick with a raised bowtie or flames in the center of it.

So if anyone has one or knows somebody that has one can you let me know if I'll see a big enough gain in mileage to warrant the time and money?