the AC belt is easy to take off and put back on, loosen the bolt in the middle of the pully and loosen the tensioner bolt, put the belt on and tighten the tensioner bolt till the belt is as tight as you want it then tighten the pully belt and that it. nothing else has to come off.
Did you put some tiny tires on it? 11MPH faster than its actually going is alot. you must have those 10" diameter wheels or something?
How bout a rear end gear change or a tranny change?
All of those could cause the speedo to be off. Usually you can change the speedo cable gears in the tranny to change what the speedo is showing compared to what its actually going. I dont know if these toyotas have them tho.