ANSWER:
When dealing in 2,000lb and 3,500lb capacity axles, hub specs are generally given as the number of bolts (lugs or studs) “on” one of four bolt circles (either “5 on 4.5”, “5 on 4.75”, “5 on 5”, or “5 on 5.5”). The easiest way is to pick a stud and number them 1 through 5 in circle (clock-wise or counter-clock-wise), take a measurement from the outside of stud #1 to the center of stud #3. You will get one of 4 different dimensions: 4.5”, 4.75”, 5”, or 5.5”.