So, say you're driving up a hill in 4th gear and the engine RPM is only about 2k, but the pedal is all the way down to the floor. You could shift to 3rd and maintain the same speed and the pedal would only be depressed about a quarter of the way down, but the engine RPM would be about 3.5k.

What scenario uses less gas?

My intuition tells me that driving up that hill in 3rd gear at a higher RPM is the better choice, because even though the engine is completing more cycles, the gearing is more efficient and thus the engine needs less gas to do the same amount of work.

So, am I right? Or am I way off

 

I think it depends. What I do is use my Air/Fuel meter to make that decision while driving. When the ECU has the engine in closed loop mode (ideal air/fuel mixture), the A/F needle swings back and forth around the 14.7:1 point. If you hit a certain combination of air flow, throttle opening and RPM, the ECU switches over to open loop mode (full rich - A/F needle reads rich). So sometimes when you floor the gas pedal, it'll go full rich and shifting down to a lower gear with higher revs lets the ECU go back to closed loop mode. Other times, usually at higher elevations, flooring it gives closed loop mode, higher revs in a lower gear gives open loop mode. I figure in closed loop mode, you are running w/ as little fuel as possible (stoich. mixture) while in open loop mode, you are pumping more fuel than necessary into the engine.

 

Yet another great post from the king himself.

I've always wondered this myself, yet always come to the conclusion that a pinned right foot uses more gas then an engine at peak(ish) tourqe.

 

I was wondering the same thing - I took a long road trip this weekend to the higher elevations of Az and there were many times when I would floor it and still lose speed so I had to down shift to 3rd and go 3500 RPMs to maintain 50 mph. And some of these hills were 3 to 6 miles long.

What concerned me was when I ran at 3500 RMPs for 5 or 6 miles and the temp would go almost to the red line but it never overheated or acted up. I guess I assumed thats what happens and its normal.

 

In my BMW, I learned pretty quickly that "accelerator pressure" uses more gas, since it has a mpg gauge. But I am not sure if it is the exact same for Toyota's.

 

Typically more pedal more fuel consumption...Less pedal higher RPM will get you there "normally" using less fuel. The further you push it down the more that goes in.

Lofregjeff: I'd take a look at you cooling sys, I would not run it too much getting that warm. That's just me though. I get nervous about 3/4 to red I don't like it getting close to that line.

 

lol @ roger ... i'm going to have to study your post a few times to make sure i understand it completely ... thanks for the in depth technical explanation, i didn't know anything about a closed loop/open loop system ... so it's not as simple as RPM/accelerator pressure, but it seems that most people agree that higher RPM and less accelerator pressure is best in most cases

x2 @ Lofregjeff ... don't let it overheat ... i pull over if i ever see the needle start to rise any little bit ... after all the work i put into my new head and head gasket, i so much rather wait for 20 minutes than have to do all that work over again ... mine will overheat when going up long steep inclines in the desert ... overheating = warped head ... pull over and wait! and even better, make sure your cooling system is all good ... a new thermostat is only like $10 and easy to do ... one of my fan blades is chipped pretty badly, that's the only easy thing left to fix in the cooling system before i really start scratching my head

 

Those gauges are a complete joke and are extremely inaccurate. They're just there because Germans like gadgets.

Fuel consumption is a combination of RPMs and throttle position.

RPMs dictate how many times the injectors fire per minute. In our trucks, RPMs range approximately from 1k - 6k. So max RPMs would consume 6 times more fuel than min RPMs.

Throttle position changes how much air goes into the engine. I've been unable to find any data on 22RE or 3VZE flowbench numbers so I can't say exactly how much difference there is between subtle pressure and flooring it. However, you can generally assume that flooring it uses two times as much fuel as half throttle although that's a pretty big assumption.

I don't think going from closed to open loop when you floor it is that big of a difference, but it doesn't help gas mileage. Most turbocharged cars including my track car run around 11:1 AFR (which is pretty damn rich) when floored and around stoich (14.7:1) when out of boost. That's only a 20% higher fuel consumption and I doubt our trucks actually get that rich when they go into open loop.

When it comes down to it though, a high revving engine will use more gas on RPMs than throttle position compared to a low revving engine (like ours) and extremely low revving diesels will use the most gas from accelerator pressure.

- - - - -
From my personal, non-scientific experience I find that keeping the RPMs down is better on mileage because I have to floor my truck everywhere as it is anyway so I don't get honked at (3VZE 4Runner).

 

Read all about the EFI system here:
- http://www.autoshop101.com/forms/h22.pdf

Have not backed up the A/F readings with real MPG figures, but that is something I would like to play with some day. But with the A/F gauge, it is interesting to watch what the ECU is doing with the engine as you drive. And most of the time, the higher RPM / less gas pedal does seem to be better, but other times it seems to be the opposite. At least based upon when the ECU is controlling the air/fuel mixture vs. when it is not. And my reasoning for being better MPG when the ECU is in control that when it is not is that Toyota must have tried to map out as many conditions of air flow, RPM, load and air temperature with ideal operating conditions to get good efficiency then the other cases they just default to some generic mode that is less efficient.

 

I have no idea whichis right. My truck's previous owner added a vaccuum guage in the dash. According to this guage, more pedal pressure in a higher gear reduces mileage. When I downshift, the guage reads better MPG as the pedal comes up some.

Don't know how accurate this is but its very easy to monitor the difference between a heavy foot and more practical driving habits. I usually ignore it and drive how I want except when I'm low on gas.

 

I have a vacuum gauge as well:


The vacuum gauge just measures the intake manifold pressure (or vacuum). They usually have higher MPG on the high vacuum side and lower MPG on the lower vacuum side, but that is basically just a label on the gauge. All it really shows is what sort of throttle opening you have, the more closed the throttle, the more pressure drop across it and the less pressure (or higher vacuum) inside the intake.

I usually use my vac. gauge to sort of tell me how much "head room" I have on the engine. That is if I am going along and the vacuum is 5" or lower, nothing is going to happen if I mash the gas pedal down, at least in that gear. Handy as you head up to higher elevations and the air pressure falls, you can figure out when to drop out of 5th gear and whether you can pull out to pass a slower vehicle, etc. And with a 22RE, the vacuum is pretty much 5" or less at highway speeds, unless you have a decent tail wind or downhill grade.

 

Actually, it is usually right. it also has an avg. mpg display that you can reset each tank. I have done the numbers several times on tanks of gas and both my numbers and both gauges on the are really close if not the same.

 

The one on my Lincoln LS works pretty well also. I have done the numbers and they match to what it displays.

My thoughts are its better to run at 3,500 and half throttle then to run at 2,200 WOT. Also knowing that running at 4K half will use more fuel than running at 3K WOT. Have to find a happy medium and it'll all work out.

 

as far as i know, the lower the rpm and the more pressure you put on the throttle, the more gas is released.

 

From my experience it is best to keep the engine in its happy zone. By that I mean the optimum rpm range no matter what gear. Often lugging in a high gear does worse on gas because the engine is actually working harder (bigger load) than when you are at higher rpms (lower gear). I read or was told that the cruising rpm range for the 22re is 2500 to 3000 rpms. This is where the best MPG performance will be achieved. With my truck (which has a turbo) If i drive with higher rpms the AFR is closer to 14.7 while just cruising than if I drive the same speed at a lower rpm in a higher gear. Also the Manifold pressure is higher and I have more of a tendancy to knock. I believe then that after an rpm higher than the happy zone is reached vacuum then begins to increase again therefore lowering mpg. (except in my sutuation)

In general you use less gas when vacuum is at a minimum. So if you hook up a vacuum gauge while cruising just adjust rpm and throttle (by gear selection) until you reach the minimum vacuum number for the speed.

 

basically keep in the powerband, which is different if u have a aftermarket cam. my 'happy zone' is 2800-4000rpms.

 

I'm not talking about the digital gauge on a lot of cars, those are generally pretty accurate. I'm talking about that needles that jumps all over the place with the mpg numbers on it on some of the older BMWs. That thing's crap.