This probably isn't the deal of the century and you can probably get petrol cheaper elsewhere BUT if you are buying unleaded petrol at Tesco then currently the super unleaded is cheaper than the ordinary stuff (well it is in Leicester anyway). Usually the super unleaded is more expensive. Both types of petrol work in all unleaded cars.
Update @ 17:21 on 20.05.2009 - I have just driven past Tesco's Hamilton in Leicester and super unleaded is still priced at 95.9p, 2p cheaper than ordinary unleaded. For any sceptics please phone Tesco's before deciding that this deal doesn't exist (0845 6779417), or simply drive to Hamilton Tesco's in Leicester.