It shouldn't make any difference whether it is a trigger or a script.
Suggest you try it with some other command (eg getdammage or name) rather than speed, to see if it really is a speed command problem.
Also, you don't really need the variable stage (although you may need it for elsewhere in the script of course.) Try
Hint Format ["%1 Km/h", speed T80]
Also try renaming the vehicle ... I don't know why T80 shouldn't work, but since it is the name of a type of unit ... its the sort of name worth avoiding. Call it T80_1 or something. Or try T80_1C for the commander.
Sorry, just rambling suggestions. ......