So I see you are still on the LiPoly learning curve.
....so I wanna limit the speed or watts....
Two different things, you can't limit Current on that controller unless you shave the shunt(not recommended).
There are a couple of things you can do now.
1)Get a controller w/ a real 3-speed limiting feature, one w/ a display that allows "on the fly" adjustment.
2)Assuming that you now realize that you don't need to be able to go faster than 40 MPH, then reconfigure your pack for more capacity and less Voltage. As I recall,you started off w/ 4S/10Ah Multistar bricks, damaged one by not having a LVC and then, somehow ended up w/ a 72V pack(you never explained how you did this). Are you running 5S/1P of 4S/10Ah LiPoly and how in the World are you charging that pack w/ an Imax B6 charger?
What is your actual LiPoly configuration now? Perhaps you can reconfigure back to 4S/1S(60V), but you would still only have 10 AH of capacity, which wouldn't help your range much.
A better approach would be, use 6) of the 4S/10Ah bricks in 3S/2P for a 48V/20Ah pack. If you then spliced the balance wires so you would have 4) 6S taps, it would be easier to balance charge, or better yet, bulk charge instead of "breaking" your pack every time you charge.
Had you thought out the LiPoly thing a little more before you jumped into it w/ both feet, you could have configured your pack w/ 4) 6S/10Ah bricks for a pack of 12/2P(46V/20Ah), like wesenwell and I use and have all the range you could want and be able to easily bulk charge w/out having to dis-connect your pack every time to charge. Your original 48V controller came w/ the correct LVC for a 12S and you could still be using it.