Wheels ET and size of tyres

[Jon Wildsmith]

yes, spacers move the wheel mating surface further inwards relative to the centre line which is negative offset

 

[KZJ90]

I'm pretty sure the Colorado 5-spokes that I used to have were -15 offset Jon... And you are certainly right about the 80 alloys being zero offset.

As the 80 alloys 'stick out' more than the old Colorado alloys on our 90, extrapolating (sp.) this logic, wouldn't this mean the -25 offset rims would be further under the arches rather than out???

I'm in danger of serious confusion here not knowing for sure which way the +/- moves the wheel

It´s marked as 16x7JJ-15 on the wheel, that "-" means only hyphen/dash (what is the right word?), not minus, so it is +15 offset/ET.
I can recommend 8x16 ET0 wheels, they make track 30 mm wider and looks better. More (negative) offset would cause problems = rubbing if not lifted.