okay so your cinq will have 13" steel or alloy wheels as standard.
your sei will either be 13" or 14"

you can go up to 15"+ though this can lead to problems with clearence especially on lowered cars.
beyond 15" and arch/body mods will be involved.

the standard wheels are 5.5 wide.
for easy fitment its best not to go beyond 6j though it is possible.

this image shows the difference in size betwen tyres/wheel size


and heres a tyre size calculator
http://www.rochfordtyres.co.uk/tyrecalc.asp

14" is generally regarded as the best compromise of size whilst keeping the handling/acceleration charcteristics of the car.

(on non sporting cinqs/sei's you must lower the car if fitting larger alloys. OE sporting springs are enough of a drop.this is by FIAT's recommendation)

now lets get onto offsets,spacers,centre bores etc.
this is where people seem to struggle(myself included lol )

fiat use a 58.1 centre bore(the hole in the centre).anything smaller will not fit bar paying a machine shop to machine the centre bore out.
anything bigger will fit but must never be fitted without the appropriate spigot ring.
this isnt a small matter,if the wrong or no spigot ring is fitted then the wheel can move around on the hub.this is extremely dangerous.

fiat use a 98 PCD(pitch circle diameter) so alloys listed with this PCD are a straight fit.
you can go to 100PCD using wobbly bolts/nuts

offset(ET) on cinq/sei is around 30-38 for replacement alloys
30.5 with a 3mm spacer at the front so an offset of 27.5 on OE alloys or for the sei abarth 36 but with an OE 10cm spacers at the front giving an offset of 26
http://www.gomog.com/allmorgan/wheeloffset.html
offset and PCD info

heres an offset calculator
http://www.1010tires.com/WheelOffsetCalculator.asp

now PCD can only be 98 or 100.thats it nothing else.
heres a list of various marques pcd's so you can see OE alloy fitments


offsets can vary but not too far.
eg on your cinq your aiming for as close to OE as possible.

i'll try and give an example.
you buy some spanking Compomotive Mo's in 14" @ 6J wide.
they have an offset of ET35 so you fit 8mm spacers on the front
bringing the offset down (you take the spacer size off the offset)

as i said im no expert and thanks to J333EVO for the above figures a snatched from one of his posts.

when fitting spacers over 5mm its really better(read safer)to fit hubcentric ones.
if you use wider than standard spacers then remember you will need longer bolts to account for the spacer width.

oh and dont forget locking nuts with those bling rims!


Handy link on wheels and fitments



http://www.rimstyle.com/pages/alloy_wheels_explained.html

care for those wheels

obvious but take care of kerbs and pot holes.more so if you run a fairly low profile tyre.
damage(excluding buckled wheels) can mostly be repaired professionally.
nice little guide here if you want to try a DIY repair on minor kerbing

http://www.autopia-carcare.com/realwhda.html


buying tyres

right so you can just drive into your local fast fit place and pay whatever they charge

or visit sites like black circles and
my tyres which offer decent prices but remember to key in fitting prices.

what i like to do is keep a raport with a local tyre place as you never know when you will need a tyre in an emergency.
so get your best online price and see what your local guy can do for a fully fitted price as even a small profit is better than no sale for them