was tested in the same manner as passenger cars before and after it, at the General Motors Milford Proving Ground. Although the Hellcat was "relatively mature" when it came to automotive performance, problems had to be rectified that included issues with the engine and new automatic transmission, weak front shock absorbers, and the position of the gun to provide more working space for the gunner. Top speed testing was done on a paved, banked oval and ride quality tests were done over specially developed stretches of bumps. The M18 also required tests of its ability to ford six feet of water, climb small walls, and ram through structures. Though it weighed about 20 tons, the Hellcat was capable of traveling at . Its power came from Wright R-975, a nine-cylinder, radial aircraft engine, the same as that used on the M4 Sherman tank, paired to a 900T Torqmatic automatic transmission. The first models of the tank destroyer were tested by the US Army's 704th Tank Destroyer Battalion. The unit had originally been trained on the M3 Gun Motor Carriage (a 75 mm gun installed in the bed of an M3 half-track). Despite its T70 prototypes requiring several improvements, the 704th had a "superlative" testing record, and the unit was later issued production Hellcats after many of their suggestions were integrated into the vehicle. The testing phase of the Hellcat proved that teamwork was an essential element of the new light tank destroyer units, and replaced the fixed, rigid structure of other units with a much more flexible command structure that allowed adapting to more complicated tasks. Design The M18's new design incorporated several innovative labor-saving maintenance features. Drivetrain maintenance was performed by removing a large access plate on the front of the hull, disconnecting the drivetrain from the driveshaft, attaching extension rails to the drivetrain mounting, and sliding the entire assembly out of the hull on the rails. For access to the bottom of the engine without having to remove it completely from the hull, it was possible to slide the engine out onto a rail system integrated with the rear hull access door. The 900T Torqmatic transmission had three forward gears, with ranges of up to 16 mph, 12 to 34 mph, and 30 to 60 mph (corresponding to cruising speeds of 12 mph, 25 mph, and 45 mph, respectively), and one reverse gear with a cruising speed of