going slower uses more gas. what??

[Isaac Zachary]

Originally Posted by maanma

Speeding up slower (longer in time) will save energy. Doubling time is halving the energy needed.

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.

[Tahoe_Hybrid]

Originally Posted by Isaac Zachary

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

[California98Civic]

Originally Posted by Isaac Zachary

That's incorrect. ... 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.

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.

[Isaac Zachary]

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.

[Blacktree]

Originally Posted by Isaac Zachary

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.

^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.

[aerohead]

Originally Posted by Tahoe_Hybrid

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

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

* 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.

[Isaac Zachary]

Originally Posted by aerohead

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.

[Phase]

Originally Posted by Blacktree

^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.

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

[Isaac Zachary]

Originally Posted by Phase

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

[aerohead]

Originally Posted by Blacktree

^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.

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.