Chose 3/8" I.D. even though most of my vehicles have 5/16" steel tubing to mate with. The 3/8" fits easily onto the auxiliary cooler hose barb ends with just a little oil applied. On the 5/16" tubing that has a flare at its end, the 3/8" hose fits over the flare with a little oil applied without cutting the inside of the hose from going over the flare end, and with a worm-drive hose clamp it seals to the smaller tube behind the flare anyway. 3/8" hose instead of 5/16" hose also results in less pressure drop for the transmission oil cooler/lubrication circuit in the winter when the fluid is cold.