Everyone says you need to calibrate as the MPG, Speedometer and other stuff are out of calibration. Can this all be done via the ELM327 mod?

If you are running the specified size tyres for the particular wheel that ford would fit then i wouldnt bother as the rolling radius difference is so marginal.

If you are running the specified size tyres for the particular wheel that ford would fit then i wouldnt bother as the rolling radius difference is so marginal.

What about 205/55 A16

And then fitting 225/40/R18 alloys?

I fitted them to my cmax Zetec as they were an option on the Titanium and had no problems what so ever, never triggered any speed cameras or anything like that and there are plenty of them around! Just fit them and dont worry about it!

Its only a 0.79% change which is nothing! http://www.etyres.co.uk/how-to-change-your-tyre-size/

205/60R16 = 632mm rolling diameter

225/40R18 = 637mm rolling diameter

Difference = 0.79%

As long as it's within 2.5% the ABS etcetera won't even know and I doubt that the dealer would be able to account for such a negligable difference.

How about the turning circle? Do I need a steering limiter?

Fit the wheels mate then get it up on a ramp and turn the steering from lock to lock checking if the wheels foul on anything. I was told needed the rack limiters for the cmax i had too so bought them but never fitted them as the wheels where nowhere near anything. My Zetec S i have now with 18's on must have the limiters fitted from the factory as the turning circle is awful!