This is strange. The following works for me on Tractors and Humvees.
I put the following line of code in a Talk trigger, followed by a cycle trigger (to make it loop). The talk trigger has a 1 second minimum. This is the code in the Talk trigger, where the humvee or tractor is named hum1:
hum1 setvelocity [15,15,1];
Every second, the vehicle is moved by the setvelocity command. Works for me.