the 2-pounder (40 mm) gun. This gun had adequate armour penetration against early war tanks but was never issued high explosive ammunition. This made the cruisers less able to deal with towed anti-tank guns, which was a serious deficiency at the long ranges of engagements during the Desert Campaign. The additional machine gun turret (as mounted on the Crusader) was no substitute for HE rounds. As the armour of German tanks increased British cruisers were up-gunned with the more powerful 57 mm Ordnance QF 6 pounder, starting with the Crusader Mk. III (an interim move pending the introduction of the next cruiser tank). Early marques of what would become the Cromwell were also fitted with the 6-pounder but the gun still did not have a satisfactory HE round. The Cromwell as planned was to have a High Velocity 75 mm gun but the gun was too large for the turret ring and so it was decided that the new Cromwell tanks would be fitted with the QF 75 mm (a bored-out 6-pounder that could take US 75 mm ammunition). The new 75 mm gun provided greater HE capability at the expense of some armour penetration and it was still adequate to deal with the majority of German armoured vehicles. Part of the Cromwell's success was its high-power to weight ratio, provided by the adoption of the Rolls-Royce Merlin engine as the Meteor, which delivered sufficient power for the Cromwell to have a maximum speed around on roads. The M4 Sherman had a top speed on roads of about . The Cromwell also had slightly superior cross-country speed and mobility. The new engine enabled the tank to be far more heavily armoured and armed than previous cruiser designs. As the Cromwell could not be fitted with the HV 75 mm, work was undertaken to produce a tank for the powerful 17-pounder anti-tank gun, able to take on the most powerfully-armed German vehicles. The Cruiser Mk VIII Challenger was developed, mounting a 17-pounder gun on a lengthened Cromwell hull in a new turret. The Challenger was an unhappy compromise, though it was popular with its crews. The cut in armour protection to allow the mounting of the larger gun meant it was not well suited to closer range engagements and it threw its tracks more often than the Cromwell. As the UK had large numbers of US M4 Sherman tanks, an extemporaneous