Sweet spot crusing speed - Page 5 - Fuelly Forums

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Old 05-25-2015, 07:58 PM   #41
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I just filled up...for comparison sake...and put 491 km on - which is mostly hwy commute - 34 km one way, and of course back...and got 7.38l/100 which is 38 Imp mpg or approx 32 US mpg...awful for such a small car. I tend to shift early, 2500 rpm, and cruise at speed limit and sometimes 10kmh below (90-100 kmh). It's better than the other car or the truck...but still, why can't it achieve 40 or better in normal driving. I'd bet a new Vette driven much the same manner (i.e. soft) would get almost the same, and be more fun. :-) Guess I'll never know.

I tried pulse/glide (engine on) on the way to work this AM for about half my commute, then a steady 90 kmh for the last half...and steady was the same but a lot less of a PITA to do. It'd drive me nuts if I did that all the time. Probably find me swinging from a rope halfway to work...if there were any trees to swing from that is.

On topic...I find around 80 kmh/50ish mph to be the sweet spot, (for all 3 - I live on flat land if it makes a difference) the Sonic will get under 6's or low 6's/100 km that way (mid-low 40's in the Imperial measure). At speed limit, pfft! Not a chance. I find it easier to drive like that on the way to work.
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Old 05-26-2015, 12:08 AM   #42
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38 Mpg is OK, it's not bad but it's not good. That's roughly what I got in my old car, but with fuel prices here, it was still pretty expensive to run. I'm assuming it's a gas engine? Unless it's one of these small intentionally efficient modern engines, then diesel is always going to offer better efficiency. I've found consistency is just as important as cruising speed now, as I said, at a constant 70 Mph on the highway,my car will do 74 MPG. I feel like a slower speed, you're not only going to take longer to complete the journey, but you may be labouring the engine somewhat.
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Old 05-26-2015, 04:40 AM   #43
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I should point out, I did the same 300 mile journey back in January and only got 62 mpg overall. The difference, besides the temperature, was that I had my car in Eco mode and also, the cars built in Sat Nav has three settings when you plan a journey, shortest, fastest and eco. I had it set to Eco and the route was quite different. Seems to have made quite a difference.
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Old 05-31-2015, 07:52 PM   #44
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Quote:
Originally Posted by JanGeo View Post
Ahhhhh I bet you don't have ethanol added to your gas like we do in the USA! I really wonder how much better my mileage would be on straight gasoline.
It would be about 10 to 15 percent better. You can find pure gas here.
Ethanol-free gas stations in the U.S. and Canada
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Old 06-03-2015, 03:43 AM   #45
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I managed 1.25 liters per hour when riding 55km/hr on a flat road with warm engine in 4th not 5th gear. 1400rpm with my small diesel. So 2.3 to 2.5 liters per 100km.https://youtu.be/aPppqlgkHXc?list=PL...dw2_CPH_DBja1z
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Old 06-03-2015, 04:45 AM   #46
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Are you sure that's correct? I think that works out at 112 MPG UK! My best was 95.9 MPG UK.
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Old 06-03-2015, 08:03 AM   #47
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Yes I am sure. Didn't do that for the whole tank but when i travel around said speed on a flat road i usually hit that number. So the cruising speed of about 55km/hr in 4th happened to be very frugal.
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Old 06-03-2015, 09:26 AM   #48
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Would 5th be too high? Revs too low? I find if the revs are too low and the engine labours, then you waste more fuel. In 4th I guess you would need less throttle which would help. But the speed is far too low to be practical for long journeys, I would need to do at least 100 kph.
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Old 06-04-2015, 02:10 AM   #49
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3rd too low and 5th too high. Both gears use more fuel.
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Old 06-04-2015, 01:24 PM   #50
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Quote:
Originally Posted by Draigflag View Post
Would 5th be too high? Revs too low? I find if the revs are too low and the engine labours, then you waste more fuel. In 4th I guess you would need less throttle which would help. But the speed is far too low to be practical for long journeys, I would need to do at least 100 kph.
Disclaimer: It's been a while since I've had to put this into words but so far I for myself am satisfied with my opinion. I would need facts and proof to sway.
Info: I ride around on my Fabia with a obdb comp. giving me a nice readout constantly of several things.

Ok let's get into the details. The turbo diesel engines have a restriction in their airway and exhaust. The turbo. When you're not using it to make power beyond what the engine would be able to put out sans turbo it's a restriction. This is what gives the turbo diesels the typical specific fuel consumption chart. With a sweet spot around 1900/2200 (IF you're using the power needed you hit that spot (Around 70 to 85% of max torque @ specific rpm). Below a certain power level and rpm (When the turbo becomes a drag) fuel consumption per effective kw goes up.

Ideally you'd run a modern turbo diesel on the rpm and power level where the gain/drag is 0 of the turbo.

Given your 100kph that you need you can check the amount of fuel it's using then and use that as the amount of fuel/power you're using to get that speed. Recalculating the fuel usage from liters/100km to liters per hour takes in effect the speed/drag and will give you a nice indication of power you're asking of the car.

Example A. Using 3 liters per 100km while riding 33kph means you're only using 1 liter per hour. Using 3 liters per 100km while riding 100kph means you're using 3 liters per hour. So putting it bluntly you're putting 3 times more load on the engine. You can keep this in mind and once you have established your max fuel usage under full throttle at a certain speed and rpm you can get a feel for how much % power it is of the total available.

Example B: When i accelerate @ 100kph in fifth gear the fuel consumption is a maximum of 10 liters per 100km and per hour (rounded number).
When i accelerate @ 33kph full throttle in 2nd gear the fuel consumption is a maximum of 30ish liters per 100km meaning 10 liters per hour. same as the one @ 100kph. Because i'm asking the same power.

Now if i factor in the normal idle load fuel usage and how much it's using when i floor it in neutral (controls @ 3000rpm) 0.5 to 1 liter per hour, we may use this as a basis for own use and see a line there indicating increased drag of the turbo and resistance in the engine at higher velocities inside.

But it's only that little per hour. So

Example A again: So using 1 liter per hour total means i effectively put 0.5 liters of fuel into power to move. When i use 3 liters per hour at 100kph i'm using 2.5 liters of fuel to move. 5 times more than at 33kph. Keep in mind i'm rounding numbers.

Sweet diesel.
You can accelerate al be it very slowly at little over the fuel use needed to maintain speed. As you accelerate and keep the liters per hour at the same level you are in fact also increasing the power. Because the speed is increased and the same amount of liters per 100km is spend quicker.

So slow acceleration with a modern diesel will lead to better results. More drastically than a gas/benzine/petrol/peut engine. Where you can not even out the restriction (gas valve) at low power levels.

When you add a air valve in the inlet things get a little different.
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