Fuel consumption rate vs. RPM

Hi all-

Wondering if anyone has ever seen a graph that shows fuel consumption rate (not sure what the unit would be....gallons/mile, gallons/minute, ?) vs. RPM. I keep hearing people talk about "keeping it below 2k RPM" to save fuel, and I get that the general idea is that the engine is more efficient at lower RPMS, but I'm curious if there is any hard data to support/maximize this effect. I.e. maybe the fuel consumption rises more steeply at 2200 RPM, so its best to stay just below 2200...but maybe that number is actually closer to 1800?

Thinking I might need to start saving for a Scangauge..

 

The lower the rpm the greater the MPG

MATH

 

Yes....but I highly doubt its just a simple straight line from idle RPM up to redline.

 

you need more fuel to get higher RPMs

there is also torque curves that come into play as well

 

This isn't true in all cases. I have some experience building volumetric-efficiency engine tunes. A fuel table usually looks something like this:


(VE Table for naturally aspirated Miata engine)

On the y-axis is the load on the engine, calculated any number of ways. On the Tacoma it is probably some combination of TPS (skinny pedal) and MAF (air flow through intake, which by physics is a function of TPS), corrected by MAT (air temp, which affects how many atoms air are in the same volume). On the x-axis (obviously) is RPM. Higher numbers in that table mean more fuel is injected.

As you can see, engine load affects fuel usage much more than RPM, and in some cases a higher RPM can be more efficient.

This example is provided as a qualitative example only, the numbers, locations, and ranges in this fuel table will differ in the Tacoma (though it probably looks qualitatively similar).

Edit:
I found this image that shows a little bit more what we're looking at. (However this one is for a boosted engine, though the principles are the same)

 

No I hear ya. At the end of the day our trucks need to be driven like grandpa in order to achieve decent mpgs, there really is no sweet spot that magically improves MPG.

 

We recently had our speed limit raised on the highway that I commute to work. While it is nice, time-wise, my MPG has taken a hit. It sucks because I am for sure out of the sweet spot, for max MPG now. But not much I can do about it, I suppose.

 

definietly not a straight line. but also you have SO many factors at play. I can make my truck consume ZERO cc's/min of fuel at 5k rpms by letting off the gas in 2nd at 60mph and allowing the motor to into fuel cut.

By buddies boosted taco will chew through 1400-1500cc/min yes.... 1.5L of fuel per min at WOT.


That stated, past 55-60mph the brick like areodymanics really take a toll on mpgs. I can do 62-65mph and get 21mpg..... bump that to 68-70 and im looking at 19ish.... 72-75 im down to 17-18, 80mph and im looking at 16mpg.

 

This calculator is not configured for Tacomas (it's from my "Mustang days") however it shows the HP needed to sustain a given mph on level ground, minimal head/tail winds, etc.--the other factor here is the engines efficiency at whatever rpm = the selected speed. If efficiency is low at that rpm mpg will suffer, if higher/high mpg will be optimal...

 

In my experience, at any RPM, these trucks guzzle fuel like beer at a frat party.

 

I get a very consistent 19.5 mpg in my retired, take truck into town and once in a while 200 mile RT to visit my brother/father, operational mode...

 

It's not that simple. There are lots of factors that affect fuel mileage. It is really more seat of the pants feel and driving experience to know how to maximize fuel mileage. Sometimes faster engine rpm's can improve fuel mileage. Even at the same RPM things like tail winds or head winds can make a huge difference. I always get the best fuel mileage on rolling, hilly or even mountain roads. Learning how to manage the accelerator pedal to get to the top with the least engine effort and coast down with enough momentum to get almost to the top of the next hill is the key.

I get the worst fuel mileage on level ground at speeds above 65 mph regardless of engine rpm. Driving 70 will show a huge drop in fuel mileage because of aerodynamics regardless of rpm. Go up to 75+ and you see another huge hit even though rpm's may only increase slightly.

 

I really don't care about mpg because my truck is a heavy pig , I try to keep my RPM's under 2500, but reguardless these trucks mpg are shit which I'm okay with

 

I am getting 19 overall and am content with that - still learning the truck but find it cruises comfortably 65 at about 2k - that's not a bad spot for cruise control. Haven't been on a real road trip yet where 75 will be the mark for big chunks - I expect mpg will take a serious hit particularly with the weight of the family and gear.

 

Sidney - I figure you have an automatic. On my 6-speed, cruising at 65mph = about 2400 rpm. Manual 6th gear = .85, automatic 5th = .72

This is based on calibrated ScanGauge, truck speedometer reads 2-3 mph high.

 

It is automatic - not based on scan - just miles and gallons. You think my guesstimates are off? Rpm too low for 65? I'll have to double check them next time out.
What is that .85 .72 - gearing? What is its significance? Thx

 

Both your and my truck have the same rear axle ratio (3.73). Difference in highest transmission gear (mine .85, your's .72) means mine turns about 20% more rpm for the same speed - so 2000 rpm on your truck = about 65 mph, while mine has to turn 2400 to go 65 mph.

Math says your guesstimates are correct. This is one reason why the automatic is rated a little higher mpg than the manual - particularly at highway speed.

 

Get a scan gauge. There is a gallons per hour (GPH) gauge on it. Drive so that you achieve the lowest GPH possible for the speed you're traveling.

Low RPM doesn't always equal best MPGs. Since MPGs is a factor of GPH over distance traveled. The faster you go at the lowest GPH will give you best economy.

Intake air temp is also plays a part. Ideling at a stoplight there can be over a .1 GPH difference just from the difference in temperature from a cool morning to warm afternoon.

Alot of outside factors play into what your ECU decides is the best fuel flow to run on. But the biggest influence you have is your right foot.

 

Thank you for explaining that @edm3rd