The key is to have the gauge and the sender calibrated to work together. So, if you have a water temperature gauge, and the right sending unit for it - you can install the sending unit where it senses transmission fluid temps, and it should work just fine provided that 1) the range of the sender/gauge are compatible with the tranny fluid temps, and 2) the material of the sender is compatible with the tranny fluid. I've used coolant temperature switches (operate similar to the sending units) in oil lines before - no problems.