[aerohead]

Originally Posted by ijames

Thanks, Nemo, for the BulletTruck link, great stuff there. I also wanted to give a link to an article at another site: Getting Better Fuel Mileage (Trucks) • Speed Talk. This site is primarily oriented towards building drag race engines but there is a lot of discussion about engine efficiency and some aero. One interesting tidbit from this article is the comment that cab to trailer front spacing needs to be under 24" and ideally 15-18". By 24" the drag has risen noticeably, and at 48" the drag is the same as two independent vehicles. Has anyone seen any other guidelines on this? That poster doesn't give a reference.

Here's one study

Here's one from GM illustrating the effect of gap-fillers