Depends on the specs. Most major fleets will govern their trucks between 62 - 65 MPH, and the speed of some may be limited by electronic engine controls which limits how many RPMs the motor can turn. Some semi trucks out there are capable of running over 120 MPH, although they're typically governed, usually for insurance reasons, at a significantly lower speed. For example, Star ran trucks with a 500 horsepower Caterpillar diesel, 13 speed transmission, and 3.55 rear end gears - a combination which makes for a triple digit truck, but the fleet was governed at 65 MPH.
a tow truck is slow
Some are. I know one of our contractors who has a Kenworth T-800 dump truck with a 525 Cummins, and that truck will move, I tell you. But a normal fleet truck isn't going to be particularly fast.
As fast as the specifications allow it to go.
truck moves fast
As fast as the truck will go... 4 LOW is the limited one < 30mph.
80 is fast.
That depends on how fast the truck is traveling.
Remove some of the hay from the truck
It gose 180
Uh, however fast the driver wants to go.
The truck was based off of a Ford super duty
about 50