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Discussion Starter #1
basic laws of physics ( or so im told ) state that to move faster an object needs more energy in a linear rate. the faster we go the more fuel is burned, I have a 2015 cruze diesel and I seem t get best mpg at 55-60mph on the highway with cruse control set. during all the testing what's the optimal speed setting
 

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I don't think the energy requirement is linear. More like geometric, roughly by a factor of 2 (squared), but a great many things factor into the energy required to move a car. Every car has a "sweet spot" for mileage. My car's best highway mileage is right around 60 - 65 mph. Anything higher and the mileage drops off. It would be interesting to see what 30 and 45 mph cruising mileages would be, but can't see where you could do that. I once drove about 10 miles at 27 mph over a slightly hilly road with the cruise control on and got an average of 65 mpg, by the Navigation System computer.
 

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Discussion Starter #3
im no physicist so I cant tell the exact formula. but I know the faster you are going it takes more energy. yah all cars have a sweet spot but im sure that after years of testing gm has found the sweet spot.might be 63 for all I know lol
 

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"Efficiency" is the sum total of a number of factors added up.

I believe aerodynamic drag increases with the square of the speed. So clearly it's a limiting factor at higher speeds. But that cuts both way - cut the speed in half and the drag becomes one forth - and at some point another factor will start to dominate such that slowing down will result in less efficiency.

One of those is engine friction. How much is it? It's what you feel when you downshift for engine braking. It's there all the time, it's just we don't notice it because normally an engine is a net producer of power. Loss due to engine friction is minimized by using a higher gear as the engine turns less for each turn of the tire.

So, I'm guessing the sweet spot is going to be around the slowest speed in the highest gear. Engine friction is minimized while aerodynamic drag hasn't become big enough to become a problem. Since you're already in top gear, there's no more efficiency to be found by going faster. But going faster causes drag to increase.

This all gets stood on it's head when we talk about hybrids. Electric motors don't have the "keep alive" need that a internal combustion engine does. Nor do they have as much friction. Their peak efficiency is at a much lower speed.
 

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Discussion Starter #5
I also try to stay in a rpm that's where the motor is most efficient. diesel is usually down in the lower rpm say 1,500-2,200 in 6th and that gets me best mpg according to the computer. at 80 mph im in a rpm range that makes less power per cycle and also increases drag and tire resistance
 

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I also try to stay in a rpm that's where the motor is most efficient.
I'm glad you said efficiency and not power band - those are two different things. I don't know enough about diesels to know where they're most efficient. But I'd imagine for all engines there's a point where the RPMs get too high for complete burning.
 

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For some reason, in my Eco I seem to be doing best @ 70 MPH on the highway in 6th and @ 45 - 50 in 5th when driving to work never really paid attention to the RPM but I think @ 2K. My driving in Cincinnati is so varied with hills construction and traffic, it is hard to find sweet spot since I don't drive hwy much.
 

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Discussion Starter #8
. diesels typically like lower rpm to do there work an are efficient there vs 5,000 rpm( this is from me and dad driving big rigs of many types.). this is true for the big diesel trucks from 3500 pickups to big rigs. that's why you see so many gears or crazy tall gears.. but in gasoline its stable enough you can rev it to 18,000 rpm ( sport bikes) but try to rev a 3500 to 9k... be ready to watch stuff blow up.
 

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Aero drag goes up as the square of your speed (regardless of gear). Horsepower requirements go up as the cube of your speed (assuming you stay in the same gear). This translates into your best fuel efficiency will occur at the slowest speed you can maintain in your top gear. I don't know what this is for the CTD, but for the gas Cruzen this means whatever your speed is at 1800 RPM. Any slower and you'll have to downshift to climb the slightest hills.
 

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Generally an IC engine develops the best fuel economy when the engine is running at the speed where peak torque is developed. During the energy crisis, truckers were complaining their fuel mileage decreased when Nixon changed the truck speed limit from 65 to 55 mph.

Same was true with my 65 Buick, at 70 mph got the best mpg at 20 mpg, slowing down to 55 decreased that to 17 mpg. Depends on gearing as well 92 DeVille and 82 P-30 motorhome does the best at 55 mph, 33 and 15 mpg respectively. But variable valve timing with a rather flat torque curve changed all this. With my 2012 MT 1.4L gas cruze, best mpg is in 5th gear at 30 mph averaging 57 mpg, but illegal to drive this slow on the interstates.

True air resistance does increase by the square of the velocity, but also affected by the drag coefficient. Direction of the wind makes a huge difference a 20 mph wind is a 40 mph difference if its a head or a tail wind. Ha, really a difference in an aircraft where at higher altitudes 50 mph winds are more common. Planes fly in the air, and not on the ground, with a cruise at 140 mph, between a head and a tail wind, either have a ground speed of 90 mph or 190 mph. And since fuel is measured in gph, get over twice the mpg with a tail wind.

Lots of variables. Poking around in town at maximum speeds of 25 mph, get better fuel economy in 2nd gear then shifting into 3rd or 4th, deals with engine loading, and in 2nd, get a lot more engine braking to save on brake wear.
 

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I can get 40 MPG at 57 to 61 MPH dòwn I 57 South to Kankakee IL. once a Month when I go see me Smurfettes in Bikinis .. I Love IT !

Whom cares about a Coefficient ?
 

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Generally an IC engine develops the best fuel economy when the engine is running at the speed where peak torque is developed.
Can you explain why that is? Running in a lower gear increases loss due to engine friction, so improved engine efficiency will have to overcome that.
 

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peak torque management is great for constant load situations such as driving water pumps. Driving on a flat road pushing though constant wind speed is all we got for constant. My car was designed around peak torque for the fact that at 6th gear 96kph is 1850 rpm.

What I have found is trying to flatten how the engine works in the various conditions. In other posts of mine you would see that I have a highly variable commute. Winds from the west pushing my car to work up and down some steep, not massive hills. on the way home same hills and the winds from the west slowing me down.

In my world winds are number 2 in my list of concerns, can not control them and with the CDI of 0.298 there are not many typical 4 door passenger cars that can beat the Cruze. The climbs are where I work hardest to keep my car as efficient as possible. and not annoying too many people behind me. I will maintain constant speed until I hit about 10L per hour of fuel consumption and if I have nobody behind me or they can pass me in a passing lane I will slowly bleed off speed and avoid regaining it until the coast down the next hill. Only a couple of times have I had to changes gears to 5th or 4th (only to avoid annoying people) typically because I have passengers or the ac is on full or lots of headwinds. 99% of the time I stay in 6th and cruising along at 96KPH (100kph if I am being stalked by a driver behind me on a single lane highway) which for my car is peak torque @ 1850RPM. However at peak torque my ECU is willing to pour in more fuel that I am willing to burn for the sake of 10kPH.

When it does come to pissing people off, I do care some, but if they really wanted to be to work early they should have left the house sooner.
 

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Discussion Starter #14
I do the same. here in Florida usa its all flat so I just cruze control at 55mph so 6th gear 1,480rpm if I remember. but yah the cruze has a very surprising drag coefficient. even I the city ill get 32mpg
 
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