XII was the first Spitfire powered by a Griffon engine to go into service. The first of 100 Supermarine-built production aircraft started appearing in October 1942; two RAF squadrons in total were equipped with the XII. Mk XIIs were manufactured from Mk Vc and Mk VIII airframes: early production aircraft had the fixed tail wheels, Dunlop AH2061 pattern "five spoke" mainwheels and small elevator balances. They had a single 85 gal main fuel tank, giving a short range of little over on internal fuel. All were fitted with the larger, pointed tip rudder. The last 45 or so Mk XIIs, were based on Mk VIIIs with two wing fuel tanks, each containing a maximum fuel load of 14 gal, and featured the larger horn balances, retractable tailwheel and undercarriage legs with torque-links, "dished" leg fairings and the stronger Dunlop AH10019 four spoke wheels. The wheels were occasionally fitted with disc-style covers. A later model IFF was fitted, replacing the aerials from the tailplane tip to fuselage with a rod aerial under the starboard wing. Another important feature of the Griffon-engine Spitfires was the entirely flush-riveted finish which was progressively introduced on all Spitfires. The single-stage Griffon engine (II or IV) gave the aircraft superb low and medium level performance, although the Mk XII's performance declined at higher altitudes: because of this all production aircraft had "clipped" wings. In comparative tests with a Mk IX it was faster at sea level, but above it had become slower. Handling, however, was considered to be better than previous Spitfire marks, and the clipped wings conferred excellent manoeuvrability through enhanced aileron response. At low altitude it was one of the fastest aircraft in the world; in one speed trial, held at Farnborough in July 1942 DP485 (now referred to as the Mk XII) piloted by Jeffrey Quill raced ahead of a Hawker Typhoon and a captured Focke-Wulf Fw 190, contrary to expectations. However pilots found it difficult to exploit this advantage in combat as German pilots were reluctant to be drawn into dogfights with Spitfires of any type below . When the Mk XII was able to engage in combat it was a formidable fighter and several Fw 190s and Bf 109Gs fell victim to it. The Mk XII's speed advantage at lower altitudes again became useful near the end of its front line service in summer 1944, when its pilots were credited with