My trick is see what speed the GPS say's and remember on the speedo where the correct speed it and at what revs.
For example with the 31's I know that the speedo is 8-9kph out, so I sit on 70 to do 78 in an 80 zone. With my other tyres I know that it's 5kph difference, so 75kph is 80.
I also see how fast traffic around me is flowing and drive slower.
You may done done for getting 5kph over, but you will never get done for going 5kph under. And like what a copper once told me, The speed limit is not a target, It is the fastest you are allowed to drive..
In saying that I absolutly love those signs that say No speed limit, drive to conditions.
