Just found this site, Click Here Thought you lot might wanna have a peek. Its really useful little java applet, will show you what difference changing your tyre sizes will make on car with a visual, plus will let you know how it affects your speed. You can also compare several at once if scroll further down the page, and will will display it all in a table.
Just used this and realised my speedo reads way out. According to this its only meant to be 0.3% out but its alot more. Anyone any ideas as to possible causes. Still have remaining warranty from garage but its quite a drive and if its a 2min fix i'll just do it myself. Cheers for any help.