As the Cromwell could not be fitted with the HV 75 mm, work was undertaken to produce a tank for the powerful 17-pdr anti-tank gun, able to take out the heaviest of German vehicles. To this end the Cruiser Mk VIII Challenger was developed, mounting a 17-pdr gun on a lengthened Cromwell hull and a totally 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 conversion of the Sherman to take a 17-pounder (as the Sherman Firefly) proved effective in providing more 17-pounder gun tanks.The Firefly accompanied Churchills, Shermans and Cromwells generally at a ratio of 1:4. The production of Fireflies greatly outpaced that of the Challenger but in Cromwell-equipped units, the Challenger was generally preferred as the Sherman had a slower road speed and inferior cross-country mobility.