I hope this thread will take some of the time and work out of the consideration of adding new wheels and tires to your XK.
I've been doing a lot of research about new wheel sizes, backspacing and offset, etc. After I install my 2.25" RR lift, I would like to go to a wider wheel (17x9) with different backspacing and offset as well as a bigger tire. The whole thing can be very confusing as to how all these sizes affect the total inner clearance as well as how much the wheels will be pushed out for a wider stance. What I have found out is that anyone who is considering wheel spacers really should think about new wheels all together. Quality wheel spacers can run about $100 per wheel. There seems to be some questions about the potential safety of having an extra 5 lugs on each wheel staying tight as well as possible premature wear on wheel bearings, etc. There are some nice wheels out there in the $175 per wheel price range. Many are teflon coated so you don't have to change wheels for winter if you live where the snow flies. Also, you can sell your stock wheels on eBay for about $300 for the set and put that money towards your project if you so choose. In my case it actually comes out cheaper to throw new wheels on than to buy and use wheel spacers and I get a more customized look. O.K. - I know that was long winded, but here are some useful tools if you are considering a new set of wheels:
First of all, a little house cleaning with stock sizes and definitions of the terminology:
The stock measurements for Commander wheels are as follows: 17x7.5" wheel with 6.5" of backspacing and 47.3mm of offset.
Backspacing: To visualize this, imagine taking your wheel off the truck and laying it FACE DOWN on the floor. Lay a straight edge across the back of the rim and measure from the straight edge down to the hub mounting surface (the flat area where the wheel meets the rotor when installed). This is your backspacing measurement.
Offset: The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel. The offset can be one of three types, zero, positive or negative. Here is a short but informative page on offset:
Here is the cat's meow of calculators when it comes to figuring all this out. It allows you to put in all of the criteria of tire size, wheel size and wheel offsets to make a side by side comparison including a visual of both wheels side by side with measurements and every spec you could possibly hope for is included!.
***The only other thing you would need to take into consideration is the difference in backspacing. The stock backspacing is 6.5". If you were thinking of buying a wheel with 4.5" of backspacing, you would be pushing the wheel out an additional 2" on top of what the calculator says. On the other hand, if you were buying a wheel with 2.5" of backspacing, you would be moving the wheel in by 2" and subtracting that from what the calculator says.
Also, here is a link to another thread on here that goes through what tires
can be used on stock as well as with different lift packages: