I have a flat bed car trailer & used it to haul some mulch a couple weeks ago. Just in-case anyone want to do the same I easily fit 3 yards of Mulch on the trailer and one in the truck. In a week or so I'll need to make another trip. I'll try 4 yards on the trailer. In this picture I already took a couple removed some mulch.

 

I can get 2.5 yards maybe 3 in the back of my truck.

That trailer with some sides could easily fit 4 yards worth.

 

Man I hate shoveling that stuff. I had 2 yards in the truck with the big toolbox in the front taking up 2 feet. I used a pair of load bars and the tark to make a tailgate kinda.

 

easier to shovel than this

about 2.5 yards of screened top soil

 

I'm sure with sides I could get a lot more mulch. The load I had didn't even come to the edges of the trailer. I'm sure we could have put another bucket load between the wheel wells. I like using the trailer because it is easier to unload.

 

I have a 16' by 6' trailer with 2' sides and I can get 10 yards in it. I move a lot of mulch being a landscaper...