Fozy,
It's not so bad with a 4x4, as these don't wheel-spin that much. However, if you have a REALLY powerful 4x4, then the performance timing function of the Stack ST700 will obviously give a false time if the car wheelspins at all. A bit like when you used to see how fast you could get a normal speedo to register on a front wheel drive car with-out actually moving anywhere - this is why you don't want it connected to a driven wheel .