Mileage observations

ajriding wrote:
Lower rpm is usually better mpg, but there are many variables in the ECU that can affect this.
If you drove a simple two cycle engine, then yes, lower equals less fuel, but on vehicles the computer changes fueling, timing, the vacuum changes, and the gears change.

In general overdrive is not an efficient gear, the gear right under it is usually the most efficient gear in the transmission, and usually translates to the most efficient at the pump (assuming speeds are appropriate for the selected gear).
In OD you cover more ground for the same engine rpm so you may get better mpg, but when towing you may not. Many variables.

In a manual transmission, in a 5 speed for ex, 5th is an OD gear, and 4th is what is referred to as a one to on or 1:1 gearing. For every one rotation of the engine the drives shaft rotates once. This literally means that the gears in the transmission are not even used, they are bypassed, not turning. The "input shaft" is locked directly to the "output shaft", so really there is no mechanical loss from the transmission. This is why it is the efficient gear (because it really is not a gear).

Similar gearing will happen in an auto trans.

The biggest factor still is how the engine fuels at different rpms and at different loads. You may find you get worse mpg at too slow of a speed and this is the engine fueling differently. 55 is typically the sweet spot, but it is worth a few bucks extra to go a little faster when you do the math.

Gear ratios for the Ford 6R140 1=3.974/2=2.318/3=1.516/4=1.149/5=0.858/6=0.674
I tow a 11,000 lb 5er at around 68 on the interstates and it runs in 6th gear. I will down shift to 5th to longer hills but doesnt on the short ones. So maybe I if lock out 6th I might get better millage??I wouldnt ever shift. Would running in a lower gear than the computer requires cause it to run hotter?