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Old 29th September 2019, 23:48   #51
kaiser
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Quote:
Originally Posted by SideValve View Post
Always an interesting debate as to whether getting a car up to cruising speed quickly gives better economy than gently getting up to speed.

Obviously booting it drinks fuel but then you can back off sooner. Not booting it uses less fuel but then you spend longer getting into top and, as you journey takes longer, longer with your engine running & therefore burning fuel.

In theory you engine is at its most efficient when at maximum BHP (which is well up the rev range) but that's efficient as in juice vs power and once you are at the desired speed you don't need the power.
Maximum efficiency is not at max power but at maximum torque.
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Old 30th September 2019, 14:35   #52
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Originally Posted by SD1too View Post
Thanks Phil. What you are seeing there is a "snapshot" of your fuel consumption at a steady 55 mph. Now zero the display just before a cold start and tell me what fuel consumption is shown immediately afterwards.

Simon
CC was not in use and driving through the city revealed 27mpg. I will try again tomorrow.
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Old 30th September 2019, 14:59   #53
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Default Fuel consumption measurements

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CC was not in use and driving through the city revealed 27mpg. I will try again tomorrow.
Phil; the crucial point is that you zeroed the display when already driving at a constant 55 mph, crucial because:
  • This speed is known to return the lowest fuel consumption.
  • There was no acceleration involved, which lowers mpg.
  • There were no cold starts or idling involved which lower mpg to a significant degree.
All these factors are part of "normal" driving and so need to be included in the measurement to ensure that the result is a realistic average.

You don't say whether you zeroed the display whilst already driving in the city. If you did, then 27 mpg is not a valid figure either. It's a short term "snapshot" in specific driving conditions.

To be able to quote a figure for overall, long term fuel consumption, the trip computer must be zeroed before the first cold start then left alone until at least an entire tank full of petrol has been consumed. Alternatively, use the brim-to-brim method under the same conditions. I'm afraid that I can guarantee that the result will not be 42 mpg!

Simon
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Old 30th September 2019, 15:04   #54
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Phil; the crucial point is that you zeroed the display when already driving at a constant 55 mph, crucial because:
  • This speed is known to return the lowest fuel consumption.
  • There was no acceleration involved, which lowers mpg.
  • There were no cold starts or idling involved which lower mpg to a significant degree.
All these factors are part of "normal" driving and so need to be included in the measurement to ensure that the result is a realistic average.

You don't say whether you zeroed the display whilst already driving in the city. If you did, then 27 mpg is not a valid figure either. It's a short term "snapshot" in specific driving conditions.

To be able to quote a figure for overall, long term fuel consumption, the trip computer must be zeroed before the first cold start then left alone until at least an entire tank full of petrol has been consumed. Alternatively, use the brim-to-brim method under the same conditions. I'm afraid that I can guarantee that the result will not be 42 mpg!

Simon
I did write “CC was not in use”
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Old 30th September 2019, 18:39   #55
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I did write “CC was not in use”
I know. Cruise control has nothing to do with it. I'm curious to learn why you think it has. It is possible to drive at a constant 55 mph using manual control of the accelerator. The human body is more sophisticated than cruise control.

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Last edited by SD1too; 30th September 2019 at 18:41..
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Old 30th September 2019, 19:41   #56
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I never reset the trip computer, that way you get an average over many thousands of miles, and it is within 1/2 mpg of brim to brim

Brian
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Old 30th September 2019, 20:01   #57
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If you do that, then why not just record the total cumulative mileage of the main counter every time.?
Not re-setting the trip counter somewhat reduces it's usefulness, I would say!. But also you prevents you from picking up any sudden changes, quickly.

And since you have so many thousand miles of data, what do you get? and in what car?
Ahh, let me guess..... 33mpg in a ....Marina???
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Old 30th September 2019, 20:20   #58
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Cos I can be very boring :¬)

Range Rover 3.9 efi - 12032 miles - 15.8 (mostly towing a horse box)
Rover 825 sd - 36927 miles - 37.38
Rover 400 1.6 - 60655 miles - 40.52
Rover 25 1.4 - 9267 miles - 44.04
Rover 75 1.8 man - 13505 miles - 37.34
Rover 75 2.0 man - 22726 miles - 32.70
MGF 1.8 - 5245 miles - 39.71
Rover 75 2.0 auto - 38928 miles - 29.56

Various different jobs/circumstances over the years so not 100% comparable
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Old 30th September 2019, 20:20   #59
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I do mine with an XL file, amount of fuel(brimmed every time) and mileage covered, XL then works out average MPG at each fill (and other useful, to me, information).
This is the summation at the end.

Total gallons 68.32

Average MPG 41.39

Total miles 2828

Cost/Mile 14.10 Pence per mile

Total fuel expense £398.87

Bought the car 29th May 2019, last fill 22nd Sept 2019, 2.0L CDTI auto
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Old 30th September 2019, 20:37   #60
kaiser
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Originally Posted by SideValve View Post
Cos I can be very boring :¬)

Range Rover 3.9 efi - 12032 miles - 15.8 (mostly towing a horse box)
Rover 825 sd - 36927 miles - 37.38
Rover 400 1.6 - 60655 miles - 40.52
Rover 25 1.4 - 9267 miles - 44.04
Rover 75 1.8 man - 13505 miles - 37.34
Rover 75 2.0 man - 22726 miles - 32.70
MGF 1.8 - 5245 miles - 39.71
Rover 75 2.0 auto - 38928 miles - 29.56

Various different jobs/circumstances over the years so not 100% comparable
What engine in your 825? Your MGF similar to mine.
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