An example of what Willowbilly is saying , My cousin has a Kenworth with a 425 Cat and 15 over trans., 4.11 gears. My Eagle has a 425 Cat and 15 direct, 3.55 gears. RPM at 60 mph is 1750 in both. Speeds in all gears is the same. The OD trans is a bit noisier in top gear, and the direct geared one seems to pull a hill better in top gear. On a long uphill grind, the Kenworth can go down to Direct and pull like crazy, quieter than the IHC in 14th (reduced gear). Once you shift them both down below that, there's no difference. Neither gets more than 5-6 MPG grossing 120,000 lbs, so I can't say if the reduced friction makes any difference in fuel consumption. Logic would say it should, but a few percent at that high rate would be awful hard to detect.