I don't know nuthin' about no retarders!
However....I might know why your bus goes 53 MPHs...
1. Measure the diameter of your wheels in inches (not the radius, the diameter), and convert that to feet, and multiply it by pi, 3.14159. (Example: 39.25" = 3.27 feet x pi = 10.27 -- the CIRCUMFERENCE of the tires on my 22.5" wheels.)
2. Take the RPMs your engine is turning at your top speed. (Example: 2800 = the number of RPMs my bus turns at top speed.)
Multiply step 1 x step 2. (Example: 10.27 x 2800 = 28,756 = the RPMs x feet in circumference of my tires.)
3. Divide the result by 5,280, the number of feet in a mile. (Example: 28,756 / 5,280 = 5.446.
The result is the number of miles per minute your bus will travel BEFORE you take into account the gearing of the rear end.
4. Divide step 3 by the rear end ratio of your bus. (Example: 5.446 / 6.5 -- the ratio on my bus's rear end -- = 0.8378.)
That is the actual number of miles per minute your bus will travel. Multiply step 4 by 60 (minutes per hour) to get the maximum speed of your bus.
5. 0.8378 x 60 = 50.27 Miles per hour top speed for my bus. The calculation works out to be about exactly where my bus tops out.
The above assumes that your tranny's top gear ratio is 1:1. If it is something else, you would have to factor that in, as well.
I bought my bus through the NC Surplus Property System from Brunswick County Schools. It had a governor on it that limited the top speed to 45 MPH, but the bus mechanics disabled that for me when I picked up the bus. IIRC, NC restricts (or used to) school busses to a top speed of 45 MPH.
If you have a 6.83:1 gear ratio, I'd bet that your top speed is limited by that rather than by anything else.
Hope this is of use.