[cscott719@yahoo.com]

If my information is correct, the 3.90 gear was only used one year, 1955 and only on Napco front axles. The reason, I believe is that in 1955, Napco did not have a half ton axle ready for the 4x4 conversions, but they did have a 3/4 ton axle with 8 lugs. This axle was based on the HO52 housing used in the 3/4 ton trucks from the factory. It used the same Eaton drop out. The trucks came with 3.90 gears in their half ton rear ends. As a stop gap until their 1/2 ton front axle was ready, the Napco Power-Pak was shipped with a 3/4 ton front axle with 3.90 ring and pinion installed to match the 1/2 ton rear axle in the truck. They also included a set of wheel adapters, so the owner did not have to carry 2 different spare tires. In 1956, Napco had their 1/2 ton front axle ready and that was used on all 1/2 tons until 1960, when GM redesigned the front suspension and installed their own "corporate" 4x4 system. I don't think they actually built the axles and transfer cases, but they were installed on the assembly line, and were not converted 2wd pickups. Until 1957, all Napco 4x4 conversions were done by the owner of the truck and at dealers. After 1957, Chevrolet installed the Napco conversion at the factory through 1959. So, as far as I can tell from bits of information here and there, the 3/4 ton Napco front axle is the only place you would ever find a set of 3.90 gears.