Hoping that someone hear has up-to-date technical info on this wheel dimension (Einpress Tiefe, nicht war??).

From an old post - http://locostbuilders.co.uk/viewthread.php?tid=50529

Having tried to plough through the suggested references (one doesn't work these days), I'm still not sure what the exact ET value is for 7-type chassis, such as ours. Checking for the Ford applications, I noted that there was no mention of Capri, Cortina or Sierra. And the old post was from 2006.

So, I used 1st principles, and check my wheels. One set is marked ET 15. The old Rimstock (ex-Westie) wheels are surely marked, but meaningless (apart from 6 1/2J). I measured the depth from rim down to mounting face and found that both sets of wheels have the same measurement of 110mm. Does that sound right?

Yes mate , provided the rims are the same width, 6 1/2

In a nutshell, as the ET decreases, the outer rim edge moves outwards. My ST170 should have ET 49 on it, and the new rims I have fitted are ET35, so now I have some issues with the outer edge catching the wing which were not there before.

Of course, if you increase the wheel width as well, this will compound the issue.

ET relates to the position of the wheel centre in relation to the outer edge (looking at it from the circumference not towards the face of the wheel), so if yours are the same width, and the measurement to the outer edge from mounting face is the same, then they are same ET.

Bit long drawn out answer I'm afraid

Does this help?
https://tiresize.com/wheel-offset-calculator/

It's got a big explanation half way down

Thank you for your replies. I reckon there will be enough clearance for the rear wheels. Fronts might be a different matter - possible rubbing against the front wings.