Studebaker & Reo US-6 (G-630) Although the CCKW was the most numerous of the WW II 2 1/2 ton, 6x6 cargo trucks, almost 200,000 of the Studebaker-designed US-6 (G-630) were produced by Studebaker and another 22,000 by Reo from 1941-45. Their engine was the 6 cal. 320cid Hercules JXD, mated with GMC transfer case, GMC transmission, and Timken