The reason we go to a gear ratio in the 2s is to be able to run in a direct drive gear, which gives us a 4% increase on fuel mileage. On your double overdrive 18spd, 18 and 17 are both overdrives. 16th gear would be direct on this transmission and that's the gear you would want to run in for best fuel economy. I'm not really up on the C15 Cat but I'm guessing you would want to run this engine in the 1250-1400 RPM range. I'm thinking with 2:85 rears the highest speed you would want to go would be about 58-59mph and 55mph or even a little less would be even better. You can always use 17 gear for faster speeds but if you do most of your running in 17th gear its defeating your purpose of the 2:85 rears. So I guess its a matter of how fast you want to do most of your running. I think you might want 2:79s and run at 55mph in 16th gear. This seems to be a really good combination. 18th gear on this setup will not be a usable gear, only for out running the cops. Hope this helps.