Can someone explain Engine RPM's in relation to MPG?

***This is in reference to Manual Tranmissions (i dunno, maybe it applies to automatics too but i think it's different).


Alright, so i was always under the impression that the higher the RPMs the more fuel you are burning, thus the worse your gas mileage becomes. However, i've heard the side of the coin that states it's not about engine RPMs as much as it is "load". So does that mean that if i'm driving 40mph in 2nd gear at 4k RPMs @ 1/4 throttle that i'd be getting the same MPG if i were in 4th gear at 2k RPMs @ 3/4 throttle?

Is that a good analogy? Or does anyone understand what i'm saying?

 

I have had a scangauge in my last 8 or 10 vehicles and in almost every condition on every vehicle the engine gets better mpg the slower it turns. The only exception is idling in gear or just off idle up to 1000 rpm or so. My 4cyl Tacoma is the same, downshifting at any speed, uphill, downhill, or whatever, lowers the mpg.

 

More vroom vroom = more gulp gulp

 

wow..........really??? e=mc yo

 

Hmmm... I actually get better gas mileage on my 2.7 with larger tires AFTER I regeared to 4.88s. My RPMs are always higher now. I'm at 3K rmp on the freeway and still get better gas mileage. I know alot come into play when bigger tires are put on so my scenario may be a little different. However, even tho i have higher rpms, my gas mileage has improved

 

Squared!

 

You get better mileage with higher RPMs because you changed the size of your tires.

In general, the lower your RPMs, the better your mileage when dealing with stock or close to stock gear ratios (between engine and road).

 

Cause i got flamed in one forum trying to believe this. Granted, it was more of a 4cyl vs 6cyl thread. My statement was "4 cyl use less fuel than 6 cyl, therefore 4cyl > 6cyl gas mileage". And everyone lost their marbles on me saying that it takes more "load" to move a lower horsepower 4cyl than it does to push a 6cyl with more HP.

My reasoning was "more cylinders = more fuel". I mean, am i wrong? Anyways, that was slightly off topic, but we did talk about engine load and how RPMs came into play and i was told that higher RPMs going up a hill is going to use less fuel than lower RPMs going up that same hill at the same speed.

 

I think he did the gears with stock tires, then got bigger tires, which would improve over stock

 

It all depends on how hard the motor needs to work. Normally, turning higher rpms makes the motor work the hardest. But in the case of larger tires, the motor is actually working extra super hard to spin those tires so the gas mileage goes down. The lower gears make less work for the motor. Kind of like downshifiting on a mountain bike... it makes it easier to pedal and doesnt make ur legs work as hard

 

actually no. I had bigger tires for a while before I got the gears

 

With the bigger tires before the gears, my rpms were extra low and my mpgs were down

 

Most of the times this is absolutely correct. Just like my mountain bike example earlier

 

10-4

 

On a stock vehicle that is totally INCORRECT. The lower the rpms the higher the mpg. The only time you need higher rpms is if you need to maintain speed, and that uses MORE fuel. This is easily measured.

 

Generally the best mpg is around 2000 rpm.....as a starting point. Next factor is th engines load if its lugging or revved too high ...yup, mpg goes down. My stock 245/75 tires get great mpg at 60 on the speedometer(standard 4 miles off so its 56 or so... another thread though) ,around 28 mpg , but at 70 on the speedo its more like 22 mpg. The drop isnt just wind resistance , but a combination with higher rpm.Youll need to do your own testing in the end for your specific setup , but between 2000 and 2700 is where i suspect most tacoma drivers fall for best mpg. You analogy of 4k vs 2k rpm and load , id say 4th gear is better than 2nd , loading the engine is often better. Consider the air volume you move , there is a ratio of fuel to that air. At 4000 rpm , you would do that for the power , that it may be lightly loaded is irrelevent to the fuel youll burn extra just spinning the engine at that speed.

 

So might I ask about shifting? Is it better to shift under 2.5k or should i move up my shifts to 3k for the first couple of gears?

I usually shift at no more than 2.5k unless i'm in a hurry or something. Best bet to keep my shift points low?

 

no set rule , in general , shift at 2000 rpm when its flat or downhill and in no hurry.
shift at 2500-3000 when flat , normal like driving. 3500 or so if on a hill. 2000 is sort of clumzy shifting on my 5 spd manual , i like 3000-3500 , very smooth there.
Use the rpm's to your favor , th 4cyl needs to rev up some.

Consider if you shift at 2500 on a hill and end up in the next gear at too low an rpm, thats why 3000-3500 works well , no harm in shifting and being around 2500 rpm in the next gear, in fact it might be ideal range to land into the next gear.