going slower uses more gas. what??
 

i tried going slower just a little bit but it wastes more gas :confused:

whats going on here i was going normal speed gets me about 18-19 upwards of 22 but going a bit slower gets about 15-16mpg

Big engine ( > 1.0 L) and gearbox is not optimized for going (slow).

For example 2.0L I4 might be utilized better running only with 2 cylinders if cylinders were utilized normally in pairs. Sure this would need run unused cylinders valves open and separated exhaust system.

Bore is oversized to be effective.

Same problem lies in mini sized engines trying get fast. Bore size is too small and speed is achieved through high rpm.

What was the grade, and what was the initial speed vs slower speed? How did you measure and then calculate efficiency?

Physically, traveling faster takes more power, so there's no getting around that.

If the slower speed had the transmission downshift that might partially explain the difference. Generally, going as slowly as possible in top gear is most efficient.

Below a certain speed yes. Even in electric vehicles too.
If I drive my leaf 20mph it won't go as far if I was driving it 30 to 35mph.
And hybrid vehicles are even more weird than gasoline.

I too have seen some very strange behavior from the onboard guessometer, I find best economy at 45-50mph with this hybrid truck. I do wish I had the hybrid aero treatments though.

its pretty much a flat grade pretty much FLOORING it from 0 to 45 nets about 20-22


going slowly from 0 to 45 is a net loss of 9-10mpg it will read about 13-15mpg on the fuel meter whole time tell i reach speed ( 47-48mph) then jump to 23-29 on the meter


the top gear is at 30mph in 4 speed mode

EVT mode(eco mode) the top gear or ratio is at 47mph (sometimes shifts if I let off the gas) the engine is at 1050 to 1100 rpm 6th gear
i don;t think the trasmssion is messed up



Maybe the battery is too far depleted ? :(

it has a hybridized automatic transmission 4 speed launch gears 0-60 in 6.5 seconds (assuming a new hybrid battery)




6.0L engine if that matters i disabled the AFM as it's risky business keeping it enabled (replacement engine COST $6,000) or 2,000 to repair it

it's always in V8 mode....


the 2mode transmission does not down shift once in eco mode you can ride 1,000RPM to 0MPH ( DFCO )from 50mph

No engine braking, only from the regen which is very light almost undetectable ( unless on the "M4 gear" 4 speed fixed gears then you get engine braking )

Correcttion: Acceleration time does not effect energy needed for whole accelaration.

So my previous answer IS WRONG (forgive me):
Speeding up slower (longer in time) will save energy. Doubling time is halving the energy needed.

Generally best fuel economy is achieved at the slowest speed you can travel in the highest gear.

Most cars that works out at around 40-50 mph.

That's an over simple fication, and I can't explain the Leaf, since I believe it has a single speed transmission.

We've all misunderstood your testing scenario. Our assumption is that you were holding a steady cruising speed, and the slower speed yielded worse fuel economy than a faster cruising speed.

You're describing acceleration, which is a rate of speed change. Speed is a constant number, like 45 MPH.

Gasoline engines are typically much more fuel efficient (power per unit of fuel) when operating around 75% throttle and around 2,500 RPM. The engine is very inefficient at lower throttle openings (slow acceleration).

It took an eternity staring at this before everything made sense to me, but it shows power per unit of gasoline in a typical engine.

https://i.stack.imgur.com/BBWi6.png

There's hardly any penalty in efficiency by going full throttle, but plenty of inefficiency at 50% or lower throttle opening across the useful RPM range.

What we should look for is something like less than 25% (of max) torque. To minimize consumption in acceleration one must use best rpm for minimized torque.

Max torque is not to thing look for best mpg on acceleration. Maximixing torque will also maximise fuel consumption in acceleration. It is for maximal accelaration with most reasonable consumption.

I learnt best options with Fuel economy plugin for Torque app. It is very effective after use forwhile.

That's incorrect. From a physical standpoint it's the same amount of energy to accelerate to a certain speed whether you do so over a long period of time or suddenly accelerate within less than a second to your desired speed.

The difference is how close or how far from optimal efficiency is the load putting onto your engine and what RPM's are you having to rev it up to to get that kind of power.

i figured out the issue two of my spark plug cables came loose :(


idles way better and does not have random shake now and fuel trims in normal before it was way positive like +12 to +15 Long term FT

Yes, and the BSFC map redpoint posted on 8/12 in this thread is handy here to explain more precisely where those "optimal efficiency" moments or "sweet spots" in the acceleration/power curve are.

I'm glad you were able to fix it!

I was also going to mention that in my Avalon I get worse fuel mileage at 45mph than 55mph cruising on a flat road. But I get better fuel mileage at 65mph than 75mph.

As mentioned before the reason has to do with changes in engine efficiency vs. aerodynamic drag. The slower you go the less power you need to maintain your speed. But that usually makes your engine less and less efficient as your power needs get farther and farther from optimal for your engine.

Because of that there's a curve to your fuel mileage when cruising at a certain speed on flat ground. There'll be a speed that gets you your best fuel mileage. Faster or slower than that will get you worse fuel mileage.

The hack to overcome this is pulsing and gliding. By accelerating you can put your engine's RPMs and load much closer to optimal while driving slower overall, both of which give you better fuel mileage.

^THIS

Every vehicle will have a 'sweet spot' where it's most fuel efficient. The vehicle's aerodynamic efficiency will have a strong affect on that. The better the aero, the higher that sweet spot will be.

wastes
 

* All internal combustion engines will experience some road load' at which they are operating at their highest thermal efficiencies, and lowest brake specific fuel consumption ( BSFC ).
* Typically, your Tahoe would see its highest possible mpg at around a 'constant' 35-mph.
* And that's based upon being : fully-warmed up ( driven @ 50-mph for at least 50-miles ), tires properly inflated, level road, no curves, dry, calm, no slowing, no accelerating, just a steady 35-mph.
* Member AeroStealth tested his 2014 Ford F-150, Crew Cab 4X4, 6.5-ft bed, 6,000-pounds curb weight plus passengers :
- 18mpg average, stop and go city driving
- 18mpg @ 62 MPH consant-speed highway
- 36mpg @ constant 35 MPH highway
--------------------------------------------------------------------------------------
I just ran the numbers for AeroStealth's 2019 Chevy BOLT:
* 35 MPH road load hp= 5.4771, @ 4.08 KW
* 60 MPH road load hp= 17.1393 @ 12.78 KW
* 70 MPH road load hp= 24.9446 @ 18.62 KW
* 80 MPH road load hp= 35.0343 @ 26.13 KW
* 90 MPH road load hp= 47.7342 @ 35.61 KW
* 95 MPH road load hp= 55.1651 @ 41.15 KW ( 1-mph over top speed )
-------------------------------------------------------------------------------------
1) The rolling resistance power requirement is increasing arithmetically (linearly ) with road velocity.
2) While the aerodynamic power requirement is increasing geometrically, as a cube function of the road / air velocity.

I prefer to look at it as energy per mile instead of power, because with power you have to divide it by the speed to figure out the energy.

In other words, the energy per mile needed to overcome rolling resistance doesn't change (much) with speed.

But with aerodynamic drag it increases with the square of velocity.

not if you have a smaller engine. my ioniq has one of the lowest drag coefficients on the market, but mpg tanks above 55 because of the smaller engine

This seems to be very true.

In some of my older cars I could get the best fuel mileage around a constant 30mph. Slower or faster would make it get worse fuel mileage. Those cars had smaller engines and less overall power.

The Avalon has one of the biggest engines I've ever had, and it seems to get it's best fuel mileage around 50 to 55mph. Below 45 and I notice a drop. Above 60 and I notice a drop.

But I still think it's the aerodynamic drag. Even a small engine will tend to run at lower than optimal BSFC load at speeds of 60 to 65mph, or even 70 to 75mph, unless your engine is so small you have to keep it pretty much floored to maintain those speeds. If you still got some acceleration in top gear at those speeds, chances are it's not that you're passing optimal BSFC.

The exception is in vehicles that they gear way too low in order to make them peppy with small engines. Some small engine cars are geared so you don't hardly have to downshift at highway speeds anytime you need some acceleration. Ideally you want to be close to full throttle to be in the most efficient zone. But that doesn't leave much room for accelerating without downshifting. So you end up with a setup that's running at some 3,500RPM and a half throttle down the highway instead of some 2,000RPM at three quarters throttle, which would be more efficient.

The

better the aero
 

In the past, General Motors Research Laboratory warned that, unless an aero-modified vehicle received gear-matching, to return the engine to original loading, that 70% of the streamlining benefit could be lost because the engine was shifted to a less efficient island of its BSFC map.
So we'd want this specificity when discussing aerodynamics and mpg.

time rate of change
 

In the past, BMW conducted an experiment which indicated that 'jack-rabbit' starts could be more fuel efficient than long, protracted accelerations.
The issue was that ICE engines have their worst thermal efficiency during transient loading.
If one got up to speed judisciously, you could reduce the engines residence time at lower efficiency, spending more time at a higher thermal efficiency, for an overall gain.

I lived my life that way during the 36HP decades; even the XFi seems to regain momentum easily.

36-HP
 

I drove the 40-HP flat-out, and no one was the wiser!
Downhills were my friend.
The only Karmann-Ghia 'dune-buggy' in the El Paso area!:)

38-42mph is the optimal sweet spot

I got the AUX pump replaced recently this way i don't have to worry about needing to cycle off my A/C in the brutal summer it still stalls out from the accelerating from auto stop :( but still can manually start it by shifting to M4. sigh...

It is a little more complicated than that. In a frictionless environment the rate of acceleration does not matter, but in the real world the faster accelerating car averages a higher speed and therefore encounters more wind resistance throughout its trip.

Example:

Car A accelerates from 0-60mph slowly over the course of 2 miles. Car B accelerates more briskly from 0-60 mph over 1/4 mile and then travels steadily at 60mph for the remaining 1 3/4 mile.

Technically they both expended the same amount of energy *accelerating* to 60 but car B averaged a higher speed over the 2 miles and therefore expended more energy overcoming wind & rolling resistance.


Of course it still may be that the engine is so much less efficient at light loads that you still burn less fuel consuming more total energy by accelerating briskly. This is true for my Prius - for years I wasted gas (and time) accelerating gingerly whereas I now accelerate right up to the "power" zone threshold and still use less gas for the trip.