Under normal circumstances, you need to get a tyre and wheel combo that equals the OEM diameter of the wheel as much as possible. in the case of the Liberty OEM 17" wheels, they come with 215/45/17 tyres. If that were to be upgraded to 18" wheels, then the closest tyre profile to match diameter is 215/40/18 (but of course most people take the opportunity to widen the tyre too, eg: 225 or 235).
In the case of my 2.5i, the OEM wheels are smaller than the GT OEM wheels. As far as I can tell, the speedo's are calibrated the same, but at an indicated 100kph, the 2.5i actual speed (by GPS) is about 95-96kph.
By increasing the size of the tyre, you can reduce this error margin. In my case, going 45 profile instead of 40 profile for my 18's resulted in about 98kph at an indicated 100kph. You have to be careful though - you don't want to go too large or you'll be going faster than the indicated speed, which will attract the attention of the the boys in blue unnecessarily.
But this is where tools such as
tyre calculators come in. Put the details of your OEM wheel and tyre on the left and the details of your proposed aftermarket wheel and tyre on the right. You can physically see the difference and the table below will give you some info about how this will affect your speedo, etc.
If you did want your speedo to be reclaibrated, there are aftermarket devices and meter organisations that can do this for you.
ABS is designed to prevent the wheel locking up under heavy braking. The only way that a tyre choice would affect this is how grippy it is. A sticky tyre such as a semi-slick should actually prevent your ABS kicking in as often compared to a cheap road tyre that slips more easily thus locking up and forcing the ABS to rapidly release and re-apply the brake pressure to regain some traction (would be worse in wet conditions).
I was going to procrastinate, but I put it off...