The Mondeo won't grip quite so well in the dry on the 16s. But isn't going to throw a fit. And you've had the good sense to decide you'll run with the 18s until they wear out. The Mondeo actually rides surprisingly well on 40 profile 18s. But you'll be lucky to get more than 15k miles out of them. And, as you have ascertained, they are expensive to replace. I think tyre makers give car manufacturers a fantastic deal on big, low profile tyres because they will reap back much more on replacements. A bit like the way Hewlett Packard virtually gives away printers, the replacement print carts for which actually cost more than the original printer with carts in it.