Whether you are using a mechanical ATD or not the additional load on the drive pinion will be the same, I have a 5inch rev counter in a box under the stairs, purchased it new about 50 years ago but never got round to fitting it, I thought it might look a little "Promenade Percy" when fitted adjacent to the 5inch Speedometer, although I suppose the 3inch rev counter is quite acceptable, even if unnecessary on our slow revving engines.