[Phantom]

I think the best thing to do with that for is to cover the top half of the back wheels and smooth the bottom of the truck.

Now just guessing for the visor leaving an inch gap between the visor and the front of the truck to let air out at speed. Also I would raise the visor up and make it a shallower angle so the visor could help direct air over the truck and help negate the drag created from air escaping between the wind shield and visor.