I got that scanguage 2 and mounted it on my dash and if im driving down the road and turn the o/d off my mileage will drop from 17.5ish to about 14.9/15.5 no matter if im towing, snow plowing or on the highway whenever i turn the O/D off my gas mileage will get worse but if i turn it off ill also have a heck of a lot more torque which is helpful when towing and snowplowing.

 

The amount of work being done by the engine will be the biggest factor in air/fuel consumption. Take an 8% grade hill. The engine may only be able to pull up the hill at 60 mph in top gear, but coming down it can probably coast at 80 mph or better, burning practically no fuel compared to the trip up the hill even though the engine is turning a higher rpm.

Not sure if my previous analogy answered your questions or not. If you're posing the same question for an engine doing the same work under both conditions, then it boils down to engine efficiency at the two different rpm's. Lugging the engine is not going to use the fuel as efficiently as it will in its sweet spot on the torque curve.

 

It has to do with throttle position. Drive a manual around in 5th gear at 20 mph, and note your throttle position to overcome the drag resistance. Now drive it around in second gear, less throttle, less FUEL (including air). The only way to make an engine accelerate is to add more fuel than in the previous revolution. If you are driving around in to tall a gear the engine will drag down, to which you will respond by either adding more fuel, or reducing the drag (downshift). Think of it this way, if you pull a trailer with 4000lbs on it does your mileage not decrease? That's because you've increased the drag coefficient, and to compensate you need more power (ie. more fuel). You're still in overdrive, still at same engine speed, but heavier on the throttle. Starts and stops added to overdrive are like as taking off in third (with a manual) and shifting to fifth by 15mph. You have to really hit the throttle to keep from stalling out, and stay on it for acceleration.

That meters the vehicles instantaneous fuel mileage, and is programmed to "think" you have lost mileage by removing the over drive.

Overdrive definately has its place, more often than not its beneficial, but it can be a hinderance to mileage as well and the original post is proof that it can happen.

 

In the first part of your question, the answer is simply yes when operated with no load, higher RPMs = more fuel/air, but when you add load and gearing into it, it's not so simple (and others have already explained it here).

To answer the second part; well it would depend on a lot of variables but if you are speaking of just the engine alone with no load, then typically this would only happen during the time after you close the throttle and the engine is winding down.

 

Whenever I am doing under 60mph, I keep O/D off. turn it on when I go over that speed. By doing so, I am getting 345-350miles to the tank...Before when i just kept O/D on the whoel time, I was getting close to 300-305miles. Huge improvement! This is with the Edge Tuned at level 2 and volant intake with flow*******s40s to balance out the bigger 33" tires I have.

 

thanks, that helps better explain the idea

how does an engine physically consume more air/fuel when it's under load but at the same rpm? would the cylinder volume remain constant? is variable valve timing the difference?

just picking the brains of you guys with more knowledge of the subject, as it's quite interesting

thanks

 

and just so it's clear, i'm not not in disagreement with you guys

it's more of a why is the sky blue question for me

 

That one is easy, I kinda already answered it. Your intake manifold is at a vacuum. This allows your throttle body to meter the amount of air allowed in and your injectors will squirt in more or less fuel based on the throttle position. Thus you can let in more fuel/air mix at the same rpm to make bigger explosions in your cylinders which makes more power to do more work at the same rpm.

 

Yep, just think of the cylinders as being "starved" at anything less than WOT. Throttle position and thus vacuum level will determine their level of starvation.

 

now i see how the vacuum within the intake manifold plays into things--you did answer this, but i didn't understand it at the time

with this, my understanding is that the throttle controls the volume of air allowed into the intake at any position except WOT, and that if a larger load is placed on the engine, then a larger volume of air and proportionally larger amount of fuel are required to create the larger explosion required to move the piston against said larger load--so rather than immediately introducing higher rpms, the larger volume of fuel mixture introduces more energy per combustion until the energy per combustion exceeds that required to move the load, at which point the rpms would increase--fair assessment?

if i'm understanding it correctly, it's clear why lower rpms against a load would result in inefficiency when the amount of fuel expended to push the load at lower rpms exceeds the amount of fuel expended to create high enough rpms to produce the power to push the load equally (where gearing comes into play)

 

By jove, I think you've got it

 

The in town milage increase has to do with torque converter lock up. In OD the torque converter will not lock up unil 45 mph, in drive it will lock up at 40mph. Atleast in my '04 with the edge. All the in town speed limits in my town are 35 mph, so I drive 40. With the torque converter spinning I waste alot of energy slipping the converter and in fact run higher rpms in OD than in drive because the converter is not locking in OD compared to converter lock in drive.

John

 

At 45 mph in OD, the engine is so far down in it's torque curve that there is hardly any throttle reponse from opeing the throttle.
However, opening the throttle more under these conditions still uses more fuel without going much faster (covering distance).
There is the loss you are seeing in fuel mileage.
Said another way, you are not getting the use out of the fuel consumed that you would under a somewhat higher rpm and better torque production.
There is a crossover point in all this that can keep shifting as loading on the motor changes.
There is no definative answer to rely on due to the varying conditions.
45 mph in city or town driving, usually is never very far at a time unless there is a main street with no lights or stops and police to run you down for going over the limit.
A proper operating tranmission would not upshift for very long under these stop and go conditions.
That 'there' OD switch is there for your to use.
The function is restored when the motor is restarted every time.
The switch signals the PCM and not the transmission. It is an inhibit function on the PCM only. This is why the PCM cancels it on restart.