Venom speed meter is only accurate if you have tires that don't balloon. Trust me.

Another idea, not yet mentioned is what I do cause I don't want to invest yet in a Garmin, is to have someone time your beast over a known distance. You can simply calculate the average speed. Have the truck at full speed when you cross the starting line, and keep it gunned till is crosses the finish line. Hence, distance/time gives you speed!