Mileage observations

The OP has a gasoline engine that produces peak torque at about 4000 rpm. He is finding he gets better fuel economy at 2000 rpm than by running at a lower rpm in a higher gear. If you look at brake specific fuel consumption maps you can see why this might occur. In the case of a 6.7 liter diesel which makes peak torque at 1800 rpm you will definitely burn more fuel per mile if you downshift. The slight amount of power lost in transmitting power through the overdrive gears is negligible.

Yes, always OD will create more heat in the trans. When the output spins faster than the input on the trans there is a lot of stress on the gear, and why it makes heat, OD towing is not recommended unless you are going downhill or have a decent tailwind. But, this really depends on the ability of your particular trans.
Not true. When I had the 7.3 with a 4 speed and 4th being OD it ran cooler in OD than it did in 3rd. I towed every where in 4th. In 4th at 2000 RPM = 68 mph 2000 RPM was the sweet spot in that truck. I have no idea what the sweet spot is on my 6.7, havent had it long enough to fig that out. The truck had 250,000 miles on it when I sold it and the trans had never been touched other than change the fluid. Yes the EGT will be lower at a higher RPM but even with the chip I had it never got hot. 4" exhaust cooled it off 200 deg

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?

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?

hotpepperkid wrote:
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?

Must be an automatic since there is no 1:1
Yes, always OD will create more heat in the trans. When the output spins faster than the input on the trans there is a lot of stress on the gear, and why it makes heat, OD towing is not recommended unless you are going downhill or have a decent tailwind. But, this really depends on the ability of your particular trans.
Looks like you have two overdrives, but 5th will heat up less than 6th in your case.
If there is a "tow mode" button, then just activate it and let the truck do the thinking most of the time.

Running a low gear at the same speed… On the engine your EGTs will be lower with faster rpm due to more air running through, or maybe it is that less fuel is needed with each cylinder. My EGTs drop a few 100 degrees when I downshift (mine is a turbo, and I am not familiar with non-turbo temps).
On the trans you might have lower trans temp when closer to the 1:1 gear ratio, this means that the gears are spinning less. On a manual, in the 1:1 gear, the gears are not spinning at all, so the heat will just come from the engine. In your case I suspect that 4th gear will generate the least amount of heat at a normal speed.
You should not have to worry about gearing and engine heat, a good radiator should handle the engine heat, but you do need to watch the trans temps. Get a good trans temp gauge and drive according to trans temps. Heat kills transmissions.

I don't want to go into explanations, better to research this on a transmission dedicated site that is specific to your vehicle. My goal is just to make you aware that there are things to watch for.

philh wrote:

ShinerBock wrote:
A few years ago, I had to follow my father who only does 60 mph. It was annoying to drive that slow. After doing the math I saved less than $6 and arrived over half an hour later than I normally do.

23 minutes later assuming the entire trip was at 60mph.

Yeah, and my time is more valuable than money at the moment. I can make $6 in about 6 minutes. I can't make 23 minutes. It may differ when I retire, but then again I put a lot of money away so it may not. Besides, I have seen people spend more on "cheap insurance" by changing their oil way too soon (which I think is worthless) or others that spend way to much on things that I could care less about, but to each their own.

Another thing that is important is my sanity. Being from Texas where most highways are 75 mph, I don't think I can stand driving 60 mph for that long of time. Others like yourself may feel differently.

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?

Lynnmor wrote:
I tow 63 as well and find it to be not a problem with traffic, yes the ego types might have a problem. I even travel I70 & I68 in MD quite often where the crazy drivers believe in drive left, pass right. My fuel, my equipment, my safety, so my choice.

It is amazing how many people take the speed limit as a challenge instead of an actual limit. Of coarse we did have the case a few years ago where a person was charged with obstructing traffic for not going far enough over the limit and had to pay the fines.

I tow 63 as well and find it to be not a problem with traffic, yes the ego types might have a problem. I even travel I70 & I68 in MD quite often where the crazy drivers believe in drive left, pass right. My fuel, my equipment, my safety, so my choice.

Another observation on mileage. We just returned from a trip towing from San Antonio, TX, to Colorado Springs. The entire trip north I got 10.5 to 11.2 mpg towing my 7800lb TT with my F-150. Needless to say, I was very happy camper, thinking that the more I "broke-in" my truck I was getting better gas mileage. However, I was brought back to reality on our return trip home when I was getting 6.4 to 8.5 mpg. Seems that having a strong tailwind vs. driving into the wind does make a huge difference. BTW, my sweet spot towing is 63mph. I was amazed at how many TTs and 5th wheels passed me doing well over 70mph. I pretty sure most had speed rated trailer tires. Wonder if speed had anything to do with the "more that several" RV pulled over for flats?