[Diesel_Dave]

I've heard discussion over how much it helps, but I don't think I've heard of anyone advocating an open bed over a tonneau. I made my tonneau for about $15--leftover plywood and some high-gloss exterior enamel and you're all set. Not quite as pretty as a commerial one, but I'm cheap.

As far as the gear ratio, there are a couple ways to figure it out. Some trucks have it on a tag inside the glove department. Another option is to give your VIN number to a Dodge dealer and they should be able to tell you. You can also jack up the rear end, rotate the rear axle by hand, and count revolutions. The last option is to look up your transmission gear ratios, and tire revs/mile, then do the math between mph & rpms.