Wheels Size

Im looking at a set of rims for a 99 XJ. The onees i have found are 15x8 5 on 5.5 with 2.75 backspacings. I assume the 5 on 5.5 means the lug nuts but how do they get the 5.5 and what do i need to measure \
thanks

The 5.5 refers to the diameter of the lug nut bolt circle. The Cherokee wheels are 5 x 4.5", so the rims you found won't fit.

Also, the stock XJ rims have 5.25" backspacing. Rims with 2.75" backspacing would extend much too far out of the wheels wells, as well as put a teremendous amount of added stress on the unit hub bearings. Most folks who don't run stock rims seem to use wheels with 4.5", 4.25" or 3.75" backspacing.