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(Post Link) post #1 of 36 Old 04-02-14 Thread Starter
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A fuel conundrum.

I drive, on a regular basis, from my home near Felixstowe in Suffolk to my daughter's in Tonbridge Kent. It is a near as dammit 100 miles and I fill up to the max before setting off. When I get there the fuel gauge is showing 3/4 full.

Upon leaving Tonbridge I fill up again - Today £24 using Shell V-Power (same as before) and when I get home the gauge is between full and 3/4. Out of interest I fill up again and it cost £19.87 - shows the gauge is telling something relating to the truth.

The roads were totally clear each way and I drove the same each time (V6 Spider) i.e. "with spirit".

Today was the first time I had filled up when I got back but I have always noticed the fuel gauge anomaly.

Any explanations?

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Is the Suffolk>Kent trip more uphill than Kent>Suffolk?
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I had to read this a couple of times before I grasped it. Odd!

Occam's razor: it is a wonky gauge. If not then
a] Some sort of relativistic effect/warp in the space-time continuum
b] One garage is selling less-energetic fuel
c] err...

Edit after reading first reply: I discounted the uphill concept as too silly...
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Fuel gauges are notoriously inaccurate, i have had a car on the bottom line of a gauge and still managed 50 miles.
 
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Headwind?
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(Post Link) post #6 of 36 Old 04-02-14 Thread Starter
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Nah gents - for the first time today I actually filled up upon return and it was £4.00 less - not a lot I agree but enough for gauge to register.

The fuel gauge - for the last year - has always shown less when I got back.

Now then - I know East Anglia is sinking into the sea but I cannot believe that my return journey is downhill all the way.
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The fuel in Suffolk is more expensive than in Kent or vice versa?
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(Post Link) post #8 of 36 Old 04-02-14 Thread Starter
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Quote:
Originally Posted by biggreenzebra View Post
The fuel in Suffolk is more expensive than in Kent or vice versa?
Nope - I still use less coming back.

I didn't check the actual litres but £19.87 is still less then £24.00 at the same price.
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(Post Link) post #9 of 36 Old 04-02-14 Thread Starter
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Just to expand - if I do the same journey in the GTV (2.0) the fuel gauge will be more or less equal either way - filling up both ends.

As an aside - the V6 costs £8.00 more in petrol than the 2.0 for the round trip - well worth it in third with your foot flat on occasion imo.
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The fuel gauge is cursed and needs an exorcism.
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Engine & fluids still warm from the original trip on way back?
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It's air density. In the morning, it's colder so your air will be denser and that means the engine is getting more oxygen in each intake stroke. The injection is using more petrol to keep the mixture correct. You'll have more BHP so you're driving it with gusto.

On the way back it's warmer, so you have a lower air density and consequently the beast is squirting less benzina in there. You're making less BHP but you don't notice it because the roads are busier so you can't be quite as gusty.... but anyway Suffolks is more down hill so you're able to use lighter throttle anyway.

Shows the fuel gauge is reasonably accurate bearing in mind it's only a general indication of what's going on ..


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With the prevailing south westerly wind we have here in the UK if you head south then you will be pushing the wind, and on the way back you will be assisted by it. I reckon that will make a difference.

Try the same again but head 100 miles north first
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Does the fuel you are filling up with in Suffolk contain a higher proportion of ethanol?
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Just a thought but if the ambient temperature on the first drive is lower than that during the second drive then the fuel will be more dense and will show as a lower level on the gauge, then on the return journey if the temperature has increased the gauge will show that there is more fuel in the tank. Assuming that is that it is a float mechanism in the tank that feeds the gauge. It is the same principle as filling up when the temperature is low means you get more fuel. Petrol - best way to fill up your car? - Detailing World
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So far then, we've got these possibilities (and of course it could be a combination):

Different temperature
Different altitude
Different traffic density
Different starting engine temp
Different relative wind direction
Different fuel quality

Any more?
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Quote:
Originally Posted by symon View Post
Does the fuel you are filling up with in Suffolk contain a higher proportion of ethanol?
Super doesn't have that. It's just reg unleaded that is going E10
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Quote:
Originally Posted by bazza View Post
So far then, we've got these possibilities (and of course it could be a combination):

Different temperature
Different altitude
Different traffic density
Different starting engine temp
Different relative wind direction
Different fuel quality

Any more?
Different pumps? When were they last calebrated? Is there stuff in the boot that isn't there on the way back?

The phase of the moon?
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Quote:
Originally Posted by PaulR View Post
Nope - I still use less coming back.

I didn't check the actual litres but £19.87 is still less then £24.00 at the same price.

There can be quite a variation in the price per litre between petrol stations, in Shrewsbury typically £1.50 to £2 on a tankfull. If you combine that with the downhill/cold/prevailing wind factors I reckon we are nearly there.

Do you have any variation in load? Do you typically take stuff with you when visiting (like gifts or baked goods) that you do not bring back?
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I think the prevailing wind is a good starting point. I have noticed, on occasions that an windy days an outward journey has been harder going than the return. Never checked the fuel each way however. Perhaps I should.

Small changes to driving style can make a big difference. I went from Scunthorpe to Revesby (for the Lincs AO meet) last Thursday and the OBC registered 33.3mpg on arrival and 35.9mpg on getting back home. I only took it slightly easier on the way back and it made very little difference to the journey time but then there was less traffic. I realise the OBC is not that accurate but it does give an indication since any error is the same for both figures. The trip was virtually 50 miles each way so 6.83 litres going and 12.66 litres for the round trip meaning I used only 5.83 litres on the return or 85% of the outward journey. That is close to the difference commented on by the OP.

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Quote:
Originally Posted by Joss View Post
Super doesn't have that. It's just reg unleaded that is going E10
Some fuel companies now are already using E5, and they don't have to declare it.

Adding Ethanol increases the effective octane rating, so having it in some super unleaded would make sense.


Not sure I am convinced by the cooler weather argument.

On a cooler day the car will be getting more air for any given throttle opening and need more fuel to make the mixture correct, but on a warmer day you would have the throttle open slightly more to compensate if you were to travel at the same speed. So in theory the fuel usage would be the same. The only difference is that on a cooler day the engine would take longer to warm up (due to the block being colder to start with) and that would make fuel use higher.
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When my parents lived in That London we used to do regular trips to visit them. Over a period of time it was was done in several different cars; the fuel consumption was nearly always better on the return journey. I put it down to tow factors, firstly topography & secondly prevailing wind. Going south it's more long uphill stretches & shorter downhill, vice versa, of course, coming north.
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Similarly, why do you have to pay to get into Wales, but its free to escape?
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Parking on some sort of slope, even a gentle one can make a difference depending where in the tank the sender is (I used to have a 306 that cut out on right hand bends if it was in the last quater!
Traffic would be a good one, but can't imagine it having that much of an effect etc?
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