Matty, I have a set of 5 original fit 16" Icon rims that were on my 2000 yr XJ when I got it, they are shod with BFG mud terrain tyres with 11mm tread remaining. Tyre size is 245/75/16 (30.7" diameter) and I also have a matching 6th rim I bought from ebay as a spare as I heard a lot of horror stories about f'roadin with alloy wheels that never materialised.
Speedo will probably be better with the bigger tyres as, in my case it was hopelessly optimistic with the standard tyres size and almost right with the 30.7's
If you are interested in buying this ready bolt-on set then PM me but they have been off-roaded so the rims are not perfect (but perfect to improve the look of your jeep at 10 paces and for use off-road)