I have a 07 SCrew 4 X 4, and it seems like I get better mileage with the O/D turned off. When I am driving in town, it shifts way too many times.Does anyone know of a way to have it defaulted to O/D off when I start the truck?

 

With the use of micro circuits being mandatory more and more, you will find it nearly impossible to select a default switch position in heavier current draw circuits. The default settings are likely to represent the great majority of users, since any position but default will require a relay to hold the circuit open or closed. It is much preferred to have a relay to be normally relaxed to reduce wear and tear on the relay and to prevent replacement of a major component like a PCM or a control module due to a simple relay failure.
So if the great majority of people use overdrive the normal default setting would be overdrive on due to the probability that a relay will be necessary to hold open the circuit to keep overdrive off.
The use of a micro momentary ground signal such as used on your overdrive switch hence requires activation each use.

 

Is that little button really that hard to push when you start it?

 

How does one get improved fuel economy under higher rpm's ??

 

There seems to be a common misconception about fuel economy and RPMs. That being that you always get better fuel economy when your engine is operating at a lower RPM. That's only part of the equation. More important to determining fuel economy are engine load and throttle position.

So if you drive around at your lower RPM and doing so causes your engine to work harder thus meaning more throttle while driving to achieve and maintain the same speeds. Then your fuel economy will be worse.

Vice versa if you drive around and your RPMs are higher, your engine is not working nearly as hard and you don't need as much throttle to achieve and maintain the same speeds. Thus your fuel economy will be better.

 

Perhaps I misunderstood.

My experience has been that I generally get better gas mileage if I leave OD on. As they say, your mileage may vary.

 

Re-read the original post. He was talking about 'in-town' driving. Not on the hwy. Leaving OD off in town probably would result in better fuel economy.

 

so how do you increase your rpms without opening the throttle?

 

Easy - be in a lower gear. TP remains the same.

 

so, if at half throttle i turn x rpms in 4th gear travelling y mph, and i can keep half throttle and turn x+600 rpms in 3rd gear at y mph, then my engine is running faster and consuming less fuel with the same throttle opening in 3rd gear?

is it drawing less air at x+600rpms? or does the a/f ratio change?

 

I'm a bit confused here...If you're in a lower gear with the same TP then the RPMs increase. More RPMs means more firing of the injectors which means more gas consumption. So unless the air/fuel ratio changes in lower gears how exactly would one get better gas mileage with the O/D off?

 

Ok, I'm starting to think some of you did not read my entire post...

Before I begin my longwinded retort let me first explain that 5:1 is a higher ratio than 2:1 so when I say higher gear ratio it generally means your driveshaft spins more times for one revolution of the tire and a lower gear ratio means your driveshaft spins less times for one revolution of the tire.

As I've already said RPMs are only part of the equation. Generally speaking, on the highway, a lower gear ratio will give you better economy. But once you introduce a lot of start and stop driving the dynamics change. Now your engine is under greater load more often. This means that you are opening the throttle wider and more often.


If your truck is geared with a higher ratio it is easier for the engine to overcome the inertia of your truck and get it moving. This results in you not having to put your foot in it so much or for as long to get up to speed driving around town.

I am not saying you will get better fuel economy with higher gear ratios on the highway, though is some instances that could be possible depending on your final drive ratio and the speed you cruise at.

My whole point here is that a lower gear ratio can cause greater engine load at lower speeds and thus make you push your pedal harder for longer, which (even at lower RPMs) introduces more fuel and air into your cylinders (remember, your intake manifold is in a state of vacuum generated by your pistons, so yes you can allow more in there even at lower RPMs) and thus expends more fuel.

The reverse is also true, since you control the amount of fuel/air introduced to the intake manifold directly with the throttle, even at higher RPMs you can use less fuel/air to do the work if it is easier for the engine (less load) to push your truck. Meaning, yes you can spin your engine (under the circumstances I've laid out here) at a higher RPM and use less fuel to do it.

I hope this spells it out so everyone can see my point.

 

I agree with Cuisinart. I have a BMW motorcycle with a very high overdrive in 6th gear. If I'm riding under 65 or 70 MPH, I get 4 MPG better mileage in 5th than I do in 6th. If I'm riding non interstate roads at 55-60 MPH, I get about 49 MPH but if I use 6th, I only get 45.

 

when an engine lacks torque at low rpms, it's necessary to use more throttle to generate higher rpms to generate more torque to turn the taller axle gears, ie 3.42 being taller than 4.10...i'm in agreement...i also understand the premise of 'lugging' an engine vs driving it in the powerband, so i agree there too

my question isn't as involved:

does an engine draw in more air and or fuel when running at higher rpms?

or phrased another way: if opening the throttle (letting in more air) results in a engine running faster, how can a faster running engine draw in less air and or fuel?

just trying to understand the details

 

I've noticed that while in OD at 45mph, and then turning it off without moving my throttle foot, the engine revs up and I slow down a bit. Putting it back into OD I can let off a bit, and I go back to 45. I think as long as your not lugging it, you will get better mileage in OD. Prob the amount of torque your motor generates figures into this too.