Originally Posted by A certain fellow
When changing the tyre size on the GT's to 19 or 20 inch, does this alter the speed reading. I have heard conflicting stories from Ford.

As long as you stick with the correct sizing there will be minimal changes to how your speedo reads.

The best sizes to use on a BA-FG Falcon when going up to 18-20" wheels are as follows:

245/40R18 - OD 653.2mm
245/35R19 - OD 654.1mm
245/30R20 - OD 655.0mm

A lot of shops will supply 225/35R20 as they were more commonly available a few years back and the trend has stuck since. These have an OD of 665.5mm which is still legal but they're really not the right tyre for a 20x8 or 20x8.5 rim. In addition, they're narrower than any tyre supplied by Ford on a performance Falcon/FPV - the contact patch shrinks and therefore grip suffers.

With the right load rating (93 or higher should be fine on most Falcon sedans) 245/30R20 are the best option in 20". The tyre isn't stretched on the rim and it's wide enough to provide ample grip. The only problem is that there is hardly any sidewall there and you have to assume the risk of running really low profile tyres. If your tyre pressures aren't high enough, a large pothole will easily destroy a wheel.

There are other sizes but there will be issues with either the size or load rating of the tyre, eg:

225/30R20 - tyre is too small for a large car and load ratings are quite low
235/30R20 - load rating is generally too low in this size and sidewalls are tiny
245/35R20 - will not fit on BA-BF Falcons on the front, possibly FG but I am yet to confirm this.

To sum it up, spend the extra $100-200 (per set of 4) and get 245/30R20 tyres. Your rims will thank you!