I've trawled all over t'internet and the consensus is that the recommended offset range for the MK6 Fiesta (02-08) is ET38 TO ET42. However, when you look up the standard Ford wheels, they all quote the offset as ET52.5.

Are Ford measuring their offset differently to everyone else (to the wheel face, rather than the hub mounting surface)? Or is there some other peculiarity that I'm missing?

depends on width of the wheels

Not really. And that's not exactly a helpful reply.

The offset is the distance between the hub mounting surface and the centre line. When it's measured, it isn't anything to do with the width of the wheel. That only comes into the equation when you're looking at the increase or decrease in clearance to the bodywork.

The standard wheels are 6.5j, most aftermarket 16" and 17" wheels are 7. If the offset were the same on both, then the clearance would decrease by the same amount on both the inner and outer edges of the wheel.

Taking an example of wheels I've been looking at today, the offset is 42. Compared to the standard wheels, there is 4mm increase in clearance on the inner edge, but it decreases by 17mm on the outer edge.

So back to my original query, which I will rephrase in an attempt to make it clearer. Is why does the standard offset not fall into the range prescribed by the 'recommended' offset. As this effectively increases the track by up to 21mm on each side, and which will undoubtedly foul on the arches at some point. To have the same inner clearance, the offset would need to be 46, so why is that not in the 'recommended' range?

Thanks for clearing up how offset/ET is measured cause none of us know

ET38-42 is the range most people get their aftermarket alloys at, which has been derived by trial and error. Owners seeking a wider track 'stance' either for perfomance reasons, aesthetics or both.

Ford's offset set is irrelevant, they will have chosen which ever wheels for a multiple of reasons. financially(cheaper), strength or durability(warranty), stay well within the tolerances of road vehicle regulations(spray suppression) and so on and so on.

ET38 will not foul the arches even when lowered thats why they are 'RECOMMENDED'

No need for sarcasm... I was just trying to illustrate that I do actually have some knowledge regarding wheels/offset etc. Sorry if I get annoyed by an unhelpful reply to a perfectly serious and honest question. :(

That the recommended offset has been found through trial and error is both helpful and reassuring, as I don't always trust supposed experts and retailers who don't always get their facts correct.