If I were to downsize my rims from 19" to 18" , what would be the minimum safe clearance between the caliper and the wheel inner rim ?

Currently I have approx 20mm between caliper and inner rime on stock 19's

Really, you could have 1mm if you wanted, make sure youve got enough room for the wheel weights to clear. So keep it 5mm or so to be safe. 

There will be deflection when driving, cornering and braking, I have fitted rims to a girlfriends vehicle when her wheels were stolen & it was left on milk crates, they turned with no interference, no problems on a short backwards and forwards drive so we set off bcak on the 60 mile journey from her place of work in the City, she reported a knocking noise at speed, on bends and on braking, I had to fit (bodge) washers between the wheel & hub flange faces while waiting for the new wheels on insurance.

 

On that basis I would say 4mm minimum, I used 3mm sometimes 2mm clearance on race cars dependant on whether rose jointed or not.

So, in theory, 1 inch smaller wheels would be 1/2" closer to the caliper, thats 12.7 mm so 18's should leave me just over 7mm. If a wheel weight is in the area of caliper -3mm ish leaving 4mm, close but safe ?

If you have 340mm front discs, 17” wheels fitted my previous Kodiaq, so 18” wheels shouldn’t  have any problems.

As far as I can discover 340x27 on the  e boost compared to 330x23 on the regular version, if the above pic is the 17's on a 340 disk, 18's should have loads of room !

14 hours ago, Mr Grump said:

So, in theory, 1 inch smaller wheels would be 1/2" closer to the caliper, thats 12.7 mm so 18's should leave me just over 7mm. If a wheel weight is in the area of caliper -3mm ish leaving 4mm, close but safe ?

That’s not how it works sorry.

different dish profiles, offsets, wheel widths will all vary and this will mean the point of closest contact will also vary.