they do what like 80-90mph atm?
The speed of the vehicles in the convoy are directly related to the distance they have to travel from where they spawn, to where they are bringing the supplies, because they all have to reach their final destination in the same amount of time.
If each field is re-supplied every 15 minutes, then a convoy that starts its delivery 2 sectors away will travel at a much faster rate than a convoy that starts its delivery 1/2 a sector away ... within that 15 minute time frame.
That is why some travel close to the speed of light ... the map maker put the spawn start a long way away from the base/strat it was supplying.