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Motoring Madness

Community Veteran
Posts: 16,862
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Registered: 06-11-2007

Motoring Madness

Some of you will know, I drive a Skoda Fabia 1.9 SDI...  Normally I expect it to return between 55 and 60 mpg, which isn`t too bad considering when it was new, it was rated at 66mpg, and it is now 9 years old and 140,000 miles on the clock....
Just been away to the far west country for a long weekend break.... and got some surprising results....
here`s the figures...... would love to see what answers you can work out, and see if they compare to mine......
start mileage figure on speedo 138847   filled tank to brim
first part of journey 304.1 miles....... refilled tank to brim  .......36.25 litres = to 8.05555 galls
second day              154.4 miles.......refilled tank to brim.............2.14 litres =   less than half a gallon
third day & return home 258 miles.......refilled tank to brim...... 18.33 litres = 4.07 galls
closing mileage figure on speedo 139564
...
Please can you work out the mpg for each day, and then tell me what you think is going on !...
39 REPLIES
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Re: Motoring Madness

You must have been free-wheeling a lot on the second day Shutter.
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David_W
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Re: Motoring Madness

Down here in the West Country we're either back roads or the A30 with not really all that much traffic which means you would have been travelling here at different speeds which increases consumption.  Whilst here you were not stopping/starting often which helps with consumption, before finally returning and then using more fuel?
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Re: Motoring Madness

Summat wrong with your figures Capn.  Finish mileage minus start mileage = 459 [139306 - 138847], and yet the individual trip mileage totals add up to 716.5. Based on the former you did 57.32mpg.
EDIT: the first two figures [304.1 + 154.4] add up to the total mileage on the speedo, so where did the third one [258] come from?
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Re: Motoring Madness

OOps  yup... sorry copied it from the wrong receipt.... have amended the original
@strat.... Yes, I do do "some" freewheeling, but not to the extent that I get over 300 mpg on the second day ! ! !.... ( and I don`t recall doing much of it this past weekend.... plenty of hills, but the downhill "free wheeling was negated by the uphill on the other side !!! )
I was suspicious of my fuel gauge on the second day, after refilling, and doing 154 miles, yet the gauge had not moved off the FULL mark all day, so I thought I had better fill up again, in case the sender was stuck in the fuel tank, and I did not know how much I had actually used..... I was gobsmacked when the pump only gave me 2.14 ltrs.!..! !  !  !  ... and it was brim full again....
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Re: Motoring Madness

closing mileage figure on speedo 139306, is in fact closing milage for 2nd Day
Speedo at close    should perhaps be 139564

makes total of 716.5 miles for 56.72 Litres (or 12.47galls)
gives a mpg of 57.42 MPG = very good
SWMBO tends to do something similiar, but works the cost out in "Pence per mile", this takes into account fuel cost changes.
Sorry Boss, copied at the same time
Community Veteran
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Re: Motoring Madness

I do keep an eye on the "overall" miles per gallon, as it is a good indicator of the condition of the engine.... especially on a "long weekend trip" where there is a variety of motoring styles !... so, yes, overall 57 mpg is pretty good considering the mileage the engine has done.... 
Normal weekly motoring is very local stuff, and so I would expect the mpg to be somewhat less, but I don`t fill to the brim for local driving, just put a tenner in, and more when it is needed.!...
The strange 154 miles to 2.14 ltrs readings, has happened once before, just after I got the car, but it wasn`t quite such a "good" result,..... only did 174 mpg on that occasion!....
pierre_pierre
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Re: Motoring Madness

Community Veteran
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Re: Motoring Madness

Very strange figures I agree, especially the second day.  It doesn't make a lot of difference, but I'm not sure about your conversions.  Litres to gallons, I usually multiply by 0.22 so 36.25 L * .22 = 7.975 Gallons (as I said, only a slight difference, and not enough to be noticeable)
As to the incredible mpg  I really have no answer. (somebody topped up your tank, while you weren't looking?  Grin)
I'm also led to believe that allowing the car to run (without your foot on the accelerator, but still in gear) will actually use less fuel than free-wheeling, (which is frowned upon of course).
John
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Re: Motoring Madness

My calculation for ltrs to galls is divide by 4.5.....
when I freewheel downhill, I keep the engine running but it is not in gear, therefore it is only running at tickover, (about 850rpm) instead of possibly 2000rpm at 58 mph.... (obviously depends on how steep the hill is, and I never do it on any hill that isn`t straight (ish) so it obviously does save fuel, albeit a very small amount.  (this topic has been covered previously, so no further comments on freewheeling thanks..... )
somehow, I don`t think someone topped up my tank for free, when I wasn`t looking.... but nice thought !..
what I can`t understand is the first day mpg figure is about 37 mpg, and the second is over 300 mpg and the third day is about 63 mpg.....  one would expect them all to be within 5 mpg of each other.... the last days figure of 63mpg is what I usually get when I go to North Wales which is about 300 miles and 65 percent motorway/ dual carriageway driving at 70 mph....
pierre_pierre
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Re: Motoring Madness

mines not a deisel, but I will do a check tomorrow, down hill in gear I can get about 130 MPG, I being a good driver dont free wheel but for you will do a quick check, I have a OBD plun do da that tells me the instantaneous
our Willy in Austria used to love idiots who free wheeled, 
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Re: Motoring Madness

The problem with "Filling to the brim" on a diesel is that it can vary, the amount of FOAM that is generated when you use a high pressure filler will be more than a slower one - so less fuel actually taken.
Your second fillup I think is in this case, you didn't actually fill it just got a lot of foam so the nozzle stopped.
pierre_pierre
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Re: Motoring Madness

it even happens on my petrol ones, there is an antiblowback flap in the spout and I can easily get a litre in after the pump switches off
mikeyd
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Re: Motoring Madness

Quote from: shutter
when I freewheel downhill, I keep the engine running but it is not in gear, therefore it is only running at tickover, (about 850rpm) instead of possibly 2000rpm at 58 mph.... (obviously depends on how steep the hill is, and I never do it on any hill that isn`t straight (ish) so it obviously does save fuel

That's a common misconception.  If you are not in gear you will be burning some diesel to keep the engine running.  If you leave the car in gear and use engine braking your engine will actually stop injecting diesel all together.  It's safer too, and saves on brake wear.
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Re: Motoring Madness

I was told it was something to do with the fact that with modern injectors, you only put fuel into the engine when your foot is on the accelerator (or you're in danger of stalling) So running downhill in gear, keeps the engine turning over, without actually putting any fuel in (but what do I know, I'm not an engineer or a mechanic)  It all sounds suspect to me, but a couple of independent sources have claimed it.
My trip computer also has an instantaneous reading, and it says, while trundling along in gear, but not accelerating I can get 999mpg (but that just means it's  too much to calculate, and it gives the same reading when free-wheeling)
John