Denny and Mattyj, thanks for the replys. I understand all what your saying. I drive 200 miles /day 6 days a week with a 2o10 Nissan frontier with a 6 cylinder engine. On the freeways afer I hit 60 I changed gears to my 5th. At 70, I changed to my 6th. For comparison, I drove the distances in 4 th gear all the way with my tack around 3700 RPM. The next time I used 5th gear. Now taking into consideration the weater(hot or cold) I got better mileage in 4th around 20.5/mile. In 5th gear I average 19.8. In 6th gear having to downshift alot, The mileage dropped to 18. Just seemed a bit odd, but your both are right.