If you consider the Scorpion a 'tank', then this is it:
http://en.wikipedia.org/wiki/FV101_Scorpion
(by modern standards it's not, but with tracks, some armor and a 76mm gun, it certainly fits...)
It's rated speed is 72.5km/h (45mph), but its sustained record is actually over 82km/h I believe.
Otherwise, T-80 variants are known to be pretty swift. A good candidate is the Ukrainian T-84:
http://en.wikipedia.org/wiki/T-84 Has a massive engine and reportedly can do about 70km/h (43mph)