Mark - You didn't specify if this was a front or rear tire (tyres, in the UK). Bjorn is correct, they should be matched both front and rear. My method follows, I don't claim it to be ideal, if someone else has a better idea feel free to jump in.
1. Measure the CIRCUMFERENCE of all 11 of you tires. The circumference is easier to measure and more accurate than measuring the diameter. Pick the tire with the smallest circumference as your spare tire.
2. Of the remaining 10 tires, pick the two tires with the closest circumference for your front tires. Ideally, they would be the two largest circumference tires, as the front tires wear much more quickly than the rears (due both to axle loading and steering wear).
3. Of the remaining 8 tires, pick the four tires with the largest circumference for one axle (I prefer the front rear axle, others prefer the rear rear axle, the choice is up to you).
4. Mount the four largest tires on the rear axle of your choice. The two larger tires should be the outer duals and the smaller two tires the inner duals (to account for crown in the road). Match the inner and outer duals circumference as close as possible.
5. Mount the four smallest tires on the remaining rear axle. Again, the two larger tires should be the outer duals and the smaller two tires the inner duals. Match the inner and outer duals circumference as close as possible.
That is it. It is a bit of a PITA to do, but depending on how much driving you do, it should only need to be done once per year at the most. If you drive only 5k miles or less per year, you may go two or three years between matching. In all actuality, I doubt most commercial drivers even worry about matching their tires at all. However, NDT's wear more quickly and less evenly than "street" tires, so more attention to the NDT's is appropriate.
