I recognise that I'm probably telling all you old lags out there (like me!) how to suck eggs, but for our younger readers, when it comes to all things tyres, wheels, offsets and ETs, I have always found this site very useful:
http://www.alloywheels.com/tyre_calculator They started life as Rochford Tyres Ltd, which shows you how old I am
and although the site generally is now all glitzy and modern, I am amused to see the fitting charts are still in the same old font us oldies recognise instantly as being forever associated with the Amstrad PCW series word processors of the 1985-95 era... ah, what wonderful machines they were...
This site is particularly useful in that it not only compares what you propose to fit with what you already have, but it also offers loads of other combinations you may not have thought of. As far as I know, it's the only site to do this but doubtless others will correct me if that's no longer the case.
Most sites simply compare 'proposed' vs 'existing' and tell you the change in rolling diameter etc, but *this* site is very good:
http://www.willtheyfit.com/ because it also shows you graphically changes in tyre width, wheel offset and even wheel arch clearance change.
Once you've been through these sites, don't forget that just because a new tyre size is possible in theory that doesn't mean that you can actually *buy* it!!!
So a good place to do a quick check is
http://www.tyre-shopper.co.uk/ Here you can enter the proposed tyre size and immediately you'll get an idea of how widely available that size is: if you get perhaps 3 or 4 tyres listed, that's bad news. A good/popular size with plenty of choice will get you maybe 40 or 50 or more choices, and a price range to match.
You can also see at a glance if the tyres offered are VAN tyres - not so likely at baby Fiat sizes, but I had a classic once with 14" wheels and these days about all you can get in that size is van tyres - but go to 15" and you get a cast of thousands. You'll find the same dramatic increase in choice in going from 12" to 13".
Finally, apart from the obvious physical fit issues of clearing the wheel arches and the disc brakes (which is where we started
) don't forget that changing the overall wheel/tyre diameter will affect your speedo reading, and of course also the car's performance, and although a speedo accuracy is not a UK MoT item afaik, the speedo does have to be fitted and it does have to work, and arguably it will not be considered to "work" if it gives a wrong reading ("wrong" means more than 10% below true speed or any amount above true speed - the MoT testers among us will correct me on that I am sure).
Sorry I've prattled on so long. I'll shut up now.