The physics of it may have to do with dampening the bounce of the rear tires on the leaf springs alone. Just a guess. When I tack them on eventually, I'll be in a place to record the difference. I keep all my truck records in a spreadsheet and run both per-tank mpg and a running average mpg.

I'm interested in the comment on the canopy effect on milage. What's your experience? I'll be adding a canopy in the next week or so. (A cab-high Leer with Yakima rack on top).

Tedd