Your tire size can affect the speedometer reading...a larger tire diameter will need fewer revolutions to go the same distance...so you will be rolling faster than the speedo says you are.
That's OK though, as long as you remember to drive slower than the speedo says, to compensate/not get a ticket.
People have put 22's on X's...and 16-17's are std on most years models...
One caveat...the backspacing should be 5.25"...or the tires can rub, especially on up travel.
(The less backspacing, the more rub...)
Another thought..
The tires for 15" size rims are cheaper...might want to keep them if $ is an issue.
Also - do you plan on off roading?
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- TJ
2001 Xterra '03 VG33, SE 5 spd, 305/70/16's, Revolvers, UBSkidderz, Doubled AAL's, 3"SL/2"BL, winch/bumpers, skids, sliders, OBA, Snorkel, pine stripes....
Friends don't let friends drive stock.
http://www.gifsoup.com/view/501230/tj-tackling-crawlers-ridge-o.gif