Place in soundscript

65TD
{
 trigger_source engine
 pitch_source engine_rpm
 sound 700.0 6.5_idle.wav
 sound 1200.0 6.5_low.wav
 sound 1500.0 6.5_mid.wav
 sound 1650.0 6.5_mid.wav
 sound 1800.0 6.5_high.wav
}

Rename the engine soundsource to 65TD in the truck file