Looking for a clean debate here. I know there will be a lot of mixed inputs and if nobody has solid concrete evidence please don't argue over hearsay.


With that being said, im feeling like having a new in depth debate while I still have my brain screwed on correctly.


So here it goes. I'll use my truck as an example.

Im running stock gears on 31's with a 22r 5 speed. 4th and 5th gear seem pretty close. I usually shift into 5th around 50mph (if memory serves me right. Last time I drove my truck was December 24th)


I do mostly rural roads that are just straight and wide open and run 5th gear holding 60-70mph at about 3/4 throttle.


I could however, run roughly 70mph in 4th gear with higher rpm's at only half throttle.


I know im an idiot for starting this debate without true tach readings. but hopefully somebody can chime in.


I usually accelerate in 4th gear and drop it in 5th to hold my speed. After 70mph there really isn't an accelerating in 5th unless you floor it and accelerate at a rate of 1mph per mile.


SO- What determines MPG- Actual rpm's, or how far down the accelerator is pressed

 

just to start on the right track.


More throttle opens the throttle plates more to allow more airflow, ultimately to suck in more fuel. No matter how much you open the intake, the engine can only take on so many cfm's per revolution. And revolutions per minute are determined by what the final drive ratio per speed is (AKA which gear is selected for a given speed)


Also, the higher rpms, the more pumps the fuel pump is making. the more air its sucking per mile and also more fuel.


The argument here is...Engines run most efficiently with no load/the least amount of load possible.


The lesser the rpm, theoretically the less fuel it should use. But the lesser rpm at cruising speed also puts a greater load on the engine VS running through the powerband in 4th gear creates more power, therefore less load.


So am I better off running more load and lower rpms? OR higher rpms and lesser load?

 

Your trying to compare pineapples to canned corn

If one is just using the factors you have given though we all know way more things impact fuel mileage

It comes down to engine rpm the slower the engine runs the better fuel mileage.

Then so many things enter in to figuring mpg

You did mean MPG not MPH ???

 

Then all engines seem to have a sweet spot where they just get the best mileage.

then of course one must operate the engine in the rpm range .

It would be nice to have a 13 speed for our Toyotas

I would love to have a overdrive to run my 3.4 at about 1800rpm at 60 mph

 

Both.

Lower RPM contributes to higher MPG.

Throttle less open contributes to higher manifold vacuum.

Higher manifold vacuum roughly translates to higher MPG.

But, in reality, it's not that simple. In fact, it's very complicated. Those are just some guidelines to keep in mind. Keep the RPM as low as possible, and the throttle as closed as possible...at all times. That will typically get you better MPG.

 

there is more at play then 2 factors. The two big ones are RPM and IPW (injector pulse width) they follow closely hand in hand. Sure it could be less work for the motor at a higher rpms in 4th gear, but the injectors are firing more often.

 

Many more factors like wind drag and roll resistance. Wind drag is huge and its affect on MPG multiplies as speed increases. My 4runner MPG starts to drop quickly above 55mph. I know I am a little off subject and you have a good question.

 

I think MudHippy summed it up best when he said that moderate revs and near closed throttle give the best results.
I think that this is true regardless of most other variables.
Of course, in the real world, our speed and power wants and needs often override best practice.

 

In my experience I've also noticed that there is a big difference in what carbs vs early efi vs newer efi vs direct injection can produce for MPG at a given RPM/engine load. Usually on older stuff, lower revs are best. Nowadays it's more dependant on overall engine load instead of RPM's.

 

There is no doubt that the newer vehicles are, the better their fuel management is.

 

plumb a vacuum gauge into the cab, and learn how to drive for high manifold vacuum.

there is a ton of info on google about it.

in general, vacuum gauges can tell you a lot about the condition of your engine:
http://www.secondchancegarage.com/public/186.cfm