The standard factory-fitted tyre size for the Fiesta is 195/50 R15 on a 6J rim, but even these give a pretty harsh ride. The Mk5 Fiesta was fitted with 175/65 R14 on a 5.5J rim (like your spare - if they put one in!), and the new 'Eco' models and some van versions, are also fitted with the 14" wheels. The 14" size with the deeper tyre is much better at asorbing road shocks, but the braking efficiency is noticably worse with the slimmer tyre. I've tried running on the 14" tyres fitted to 6J rims and this gives braking efficiency comparable to the 15" tyres. I read that the discs and pads may be different (smaller?) on the models fitted with 14" wheels, so as not to bring the ABS in so early. I've tried wrapping a steel tape measure around both the 15" and 14" tyres fitted to a rim, and I'm measuring the rolling circumference of both part-worn tyres as 184cm.