does anyone know if i can fit 40"x15.5x20 on 20x9 wheels or is that to wide for that wheel

 

Way too wide. Rule of thumb is ideally you want the tire to be 3" wider than the wheel, with a 1" -/+ being acceptable. So I'd say 20x12 would be what I'd plan for if using a 15.5" tire...

 

that rim is to skinny but it depends on what your gonna be doing. if your goin to use it as an off road vehicle then you can use a 20x10 and you would be fine and it would be harder to pop the bead. but if your gonna use it as a street truck a 20x12 is what you would want. a 20x12 would be ok off road but you will have a better chance of popping a bead.