The spring diameter will play the biggest part. They typically range in diameter from 2.25", 60mm or 2.50". JRZ doesn't seem to list the specs for the spring diameters on their site.
Determine the coilover spring diameter.
Measure the diameter of your present shock shield or body.
Measure the distance between the wheel and present shock shield or body at closest point.
Subtract half the diameter of the spring minus the present shock shield diameter from the distance between the wheel and present shock and you'll have the available clearance for wheel/tire.
Only an example, not actual measurements
Coilover spring = 60mm
Existing shock = 50mm
Wheel to existing shock = 20mm
(60-50)/2 = 5mm
20-5 = 15mm (available backspace based on existing wheel specs)
Using the above, you could go an inch wider with the same offset and have 2mm to the spring.
Or, wait for the coilovers to arrive and take direct measurements to the wheel/tire from the coilover spring.
I run an 18x11 that sits flush and can tell you there isn't much available room on the inside.
