Hi Albert, the reason I use two at a time is that it keeps the vehicle level so giving an accurate reading. I've proved this several times by taking a reading then swapping the units over and repeating the readings. You can only use one unit at a time but you need to put a piece of wood of the same thickness under the opposite wheel of that axle to keep the vehicle level or you get inaccurate readings. Front to rear isn't critical but across the axel it needs to be level to get an accurate reading.
D.
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