They both are.
The exact same thing as the first drive axle.
A tandem truck usually refers to the amount of axles on the trailer or tractor. A single drive axle on a tractor would be referred to as a single, or the same for one axle on the trailer. I have seen it referred to the amount of tires on the end of an axle. When there are two tires on the end of the axle, that would be referred to as a dual, not a tandem.
The "tandem" part means it has two drive axles, although it is possible for a tandem truck to be a 6x2 vehicle, and have only one of those be a live axle.
Tandem by definition is two. So tandem axle is two axles. Example would be a tractor trailer having two axles next to each other is a tandem axle.
Five
Three - the two drive axles, and the steer axle.
Tandem axles are connected by spring Multi axle trailers Shipyard Transporters
The length from the middle of the steer wheels to the center point between the drive tandems (for a tandem truck) or center of the single drive axle (on a single axle truck).
The average tandem axle truck can carry about 12 cubic yards of soil. Generally, a tandem axle truck has three axles.
Both of them.
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There's no such thing as a one axle truck. There has to be a minimum of two axles. "Single axle" if a term referring to a truck with only a single drive axle, and doesn't count the steer axle. Same with "tandem axle", "tri axle", "quad axle", and "quint axle".