Thanks for that, I understand the 'aspect ratio'
I just used this to check the rolling radius
http://www.club80-90syncro.co.uk/Syncro_website/TechnicalPages/TRC , I input 205/70/16 VS 235/85/16.
Theres not a hell of a lot of height difference in the actual wheel centre which would determine how far off the deck the diff is....
Regarding tyre prices, I have been using tyres up at 500 mile intervals & they are currently £140 to replace so getting mega mileage out of a set of four at less per unit isnt so bad
EDIT: the link doesnt seem to be working, I Googled tyre rolling radius converter so anything that search brings up will tell the same story