The Challenger can make 75 MPH without the turret fitted.
Most tanks are faster without the turrets fitted, as it makes up 50% of the tank's weigh due to the bulk of the armour being placed there.
As an ex-tank crewman, I can tell you that all tanks have a finite speed determined by their suspension system and tranmission control systems.
Unlike automobiles, tanks become harder to control the faster that they move. Things such as track whiplash, track innertia, suspension rock, etc. place stress on the undercarriage.
All the world's fastest tanks, from the 1930's such as the US M1929, M1931 and M1932 (T3), Soviet BT series, British A13, etc. all had one thing in common.... low weight and therefore low inertial mass narrow tracks.
At 50 MPH the top of the track run is moving forwards at 100 MPH, which means that to the driver struggling to keep the tank straight on course, he has to fight against the innertial mass of several tons.
The other problem is that the goveners are put in for a reason.... to stop track innertia induced downhill runaways. If you can remember the old friction (flywheel) driven kids toys of old, that's how the effect builds. So taking the govener off of a M1 would result in an uncontrolled runaway situation on even the shallowest of long downhill inclines when travelling at more than the governed top speed, with the same force of a runaway train.
The old 1970's Scimtar, I was a gunner / wireless op./ crew medic on, had a top speed of 60 MPH on the level (dry) and 50 MPH fully tooled and kitted up.
Our armour was 61mm. front, sides and rear.
Powerplant was a 4.2 litre Jaguar car engine and the suspension was Christee type.
Armament was 1 x M1919A2 and 1 x 30mm. Rarden hyper velocity auto-cannon
Overall weight was around 8 Tons.