I can see nearly a centermeter of no wear pattern to the top of the pully so i assume that its not travelling to full potential maybe?
Well then you mosy definately have way too short a belt in there then, or the belt is not travelling to the outer edge because you can only hit 57km/h?
Mark the reat pulley too with a marker,could be the belt isnt going all the way in to the bottom of the pulley at the rear. Or you have one of the restricted rear pulleys with the raised lip inside to stop tbe belt travelling fully in? Have a look, its easy to see, will be a raised bit on the face of the torque drive.
If it is a stock belt from the factory it should be fine, tho if you swapped it out it may well be shorter. Belt should be 729mm long round the outside edge. I ran a Malossi Kevlar belt last year at 736mm long which gave me 4-5mph extra top end but then i went back to stock once i fitted a shorter boss which still gave me the same top end.