This paper discussed the flight test of a modification to the Cessna 337 Skymaster to improve its range and altitude performance by the addition of three foot wing extensions which contain fuel tanks with 20 gallon capacity. In addition the takeoff gross weight was increased from 4700 lbs. to 5120 lbs. The tests were successful with the modification receiving its Supplemental Type Certificate (STC). The only problem encountered required a minor modification to the static ports of the aircraft to achieve an accurate airspeed calibration.