This isn't a problem with the convoys, it's a problem with the terrain builder.
The terrains are being built with 10 mile roads instead of 8 mile roads, that's what's causing it. At 8 miles, the trucks would travel around 40-45 mph. At 10 miles, they travel at 60 mph. If the road is stretched to show a longer road, or extraneous waypoints are included, which extends the overall length as well as causing backtracking convoys, the trucks pick up speed.
The solution is to keep the roads at 7 to 8 miles long and make sure there are no hidden extra road segments/waypoints.