Simply set up a drag of X feet, have someone time the run, and calculate it from there. For best results (to lessen the effect of latency of human reaction on the speed/time), set up a longer drag.

Measure out say 100 feet. Give yourself Y feet of initial run before the drag. Time it, and then calculate it.

(LENGTH_FT/TIME_SEC)*(1(mi)/5280(ft/mi))*(60sec/1m)*(60min/1hr)

So if you've got a drag of 100 feet and your truck finishes it in 2.25 seconds:

(100/2.25)*(1/5280)*(60/1)*(60/1) = 30.3MPH