Tyres for 18"

[tucpal]

if your origanl tyres were 205. 60. r16. then you would need 225. 45. r18
this will keep the diameter that was 65.24cm and increase it to 65.97cm
and that is a good match so you speed error will be minimal
before with old tyres 30mph
with new tyre size 30.3mph

now if you old tyres were 205 60 17 that would be a different story
you would need 225 50 r18
so at 30mph your new speed would be 30.2mph

have a look at this lewis

https://www.google.co.uk/url?sa=t&rct=j&...7201,d.ZGU


also you would have to compare the same brand tyres
as baz said you can not compare different brands for price

you could try maxxis

http://www.google.co.uk/aclk?sa=l&ai=CMu...rodlisting

as for speed rating you dont want anything under the max speed of the car