Well, this just defines, how sharply you can turn at low speed. Here are some values again:
0.3 = Very tight. Use this just for fun.
0.5 = A bit tighter than average.
0.7 = A good average value, although not the realistic one.
1.0 = Realistic value for common cars.
2.0 = For trucks and other long cars, can't get very sharp turns with this.