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Well, actually, you have to put all three together... 205 x 0.4 x 2 (sidewall counts twice in height) then divide by 25.4 and add rim size (17") for the height 23.45+"... but yeah, when comparing similar tires with similar widths, the middle number is what really counts.
My guess as to why Ford is okay with the big discrepancy (3%)... having the shortest tire on the 17s helps improve gearing for acceleration, partially negating the effects of wheel weight on straight-line performance and making the "sporty" Fiesta feel even more so... and it's going to be doing nearly 100 rpm more than the "high economy" variant on the highway at speed.
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