Very nice photos LS1380, some very nice cars over there!
But they don't prove that 13's with tyres are the same rolling diameter as 10's. I've double checked that my calculations aren't out, see here:
http://www.tyresave.co.uk/tyresize.html . Try putting in 175/50/R13 compared to 165/70/R10. I didn't use that site last night, I just calculated it - rim diameter + (2 x profile (which is a percentage - so 2x0.7 or 0.5) x width). So, for example, (13 x 25.4) + (2 x 0.5 x 175) = 330.2 + 175 = 505.2mm. That website gives the same numbers.
Photos that prove that 175/50/R13's are the same outside diameter as 10's would be photos with a tape measure draped across each tyre, giving the same dimension, or photos taken from beside stacked tyres. Photos taken from above will have parallax errors. Photos from the side should capture about the same angle of the tyre, and should show differences in size more clearly.
It is just a shame that we can't get 175/45/13's and 165/55/12's - they're within a couple of mm of identical rolling diameter to the 10". Although I doubt there'd be any/much difference in bearing wear, given that's probably more a function of the width of the tyre, not so much the diameter (as mentioned by Jan/Ashphalt), and the ride would be worse.
