The "drive- thru" measurement is the shortest distance between the arms when in the lowered position. To determine the "drive-thru" width of your vehicle, measure the outside distance of the widest wheel assembly that will need to "drive-thru" the arms (when the arms are in the lowered position).
A dual wheel truck DOES NOT need to completely "drive-thru" the lowered lift arms to be properly positioned for lifting. Only the front wheels of the dual wheel truck need to "drive-thru" the arms for the truck to be raised. Most wheel assemblies are NARROWER than the body of the vehicle.
Width between columns is important when you are working with WIDE vehicles.
"Drive-thru" measurement is important when you want to drive through the lift (arms in the lowered position) to store a vehicle "behind" the lift.
With the arms in the "raised position", the "drive-thru" measurement is the width between the columns. (Assuming the widest (top) part of the vehicle will not contact the arms in the narrower raised position).
The "drive-thru" measurement of a four post lift is measured from the inside of both the cross beam lock covers. This "drive-thru" measurement should be used when determining the maximum side to side outside vehicle WHEEL dimensions that can be safely positioned on the runways of the lift.
The "drive-thru" measurement of the lift (with the ramps in the full "UP" position) would be the distance between the columns. This is an important measurement when determining the maximum width of a vehicle that can be stored under a four post lift in the raised position.