Hi again msford,
Based on what you have written I'll conclude that the later model Mondeos were fitted with the 45 profile tyres on 18 inch wheels AND the electronics determining speedometer readings were altered to provide proper readings (your 97 kph when 100 kph is being shown is the sort of figure I would expect - especially for tyres that had some wear).
That being said, if the original tyres were 45 profile and the replacements are 40 profile AND the tread width remains unaltered then the difference in diameter is as I previously indicated. A change of profile from 45 to 40 would have some effects that you might not want:
1. At an indicated 100kph your speed would be 93.5 kph (based on your 97 actual when 100 is indicated currently)
2. Service intervals will be indicated after 14,500 km approximately
3. Tyre life will be lower (assuming the tyre compound is the same for both tyres)
4. If my recall about wheel/tyre diameter alterations is accurate then the car would be considered unroadworthy owing to the total diameter alteration.
One of the problems we have in this country is that Ford Australia's Customer Care people, generally, are not technical and tend to refer technical questions back to the dealer, and, in my experience, dealers are not yet adequately trained about the various combinations that imported fords can be equipped with.
I had hoped the Ford ETIS site (
www.etis.ford.com) would provide information about the OE tyres, but no. Yes for OE wheels, but no information about tyre fitment.
I'll admit that the information from your tyre placard is interesting. I know that the later models of Mondeo have a much enlarged information panel compared with our car. Ours is shown in the attached picture.
For the wheel/tyre combinations the unloaded diameters are 642.9mm (215/55R16), 643.3mm (235/45R17) and 645.2mm (235/40R18). Note that all these figures lie within 2.3mm of one another ie the maximum radius variation is 1.15 mm and I might comment that the radius of the wheel/tyre combination varies much more than that as wear takes place.
It is also interesting that the placard indicates which tyre is applicable to which engine, because the column to the right of the tyre sizes shows engine displacement information (2.3 litre NA petrol, 2.0 litre Turbo Diesel and 2.5 litre Turbo Petrol). Broadly speaking these engines relate to the basic car (2.3 litre) the turbo diesel models (usually Zetec standard) and the XR5, but these names are now old hat.
And sorry ope126 but your understanding isn't quite correct. MA series vehicles could appear with 55, 45 and 40 profile tyres. I think that the later models had even more variation.
Incidentally, if you want to work out the unloaded diameter of a wheel/tyre combination then you use:
(Tyre width in mm x tyre profile expressed as a decimal fraction of 1.0 x 2) + (wheel diameter in inches x 25.4)
hence a 235/45R17 combination gives (235x0.45 x 2) + (17 x 25.4) = 643.3 mm
If you do google search this sort of information is readily available, in fact I think it can be found on the forum. A useful site is:
http://www.rimsntires.com/specspro.jsp
Cheers