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I drive 180 miles on £13 diesel getting 61mpg, drive another 70 miles using 9 litres and mpg drops to 50 mpg, why?


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I really like to work out what mpg I get from my car but certain things really confuse me.

 

Today for example, I was doing a 180 mile journey so topped up with £15 of diesel in my car to get full to the brim again ( takes approx. £44 to fill from empty to full at current prices)

once I drove 180 miles I look at my fuel gauge and see the gauge was slightly above where it was before putting in £15 earlier on so by my calculation I must have used around 13 litres of fuel which would be around £13, which is approx. 62 mpg.  however if i was to drive about another 70 miles it will probably be at the half fuel point, and from the half fuel point takes 22 litres. 

if I drive the 70 miles then that would be half way on fuel that would only be 250 miles from 22 litres which is 51 mpg. but from the first 13 litres it shows I got 62 mpg, so by driving extra 70 miles my mpg reads 51 mpg despite being on same roads e.t.c. ????

 

 

why is this ?

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The tank is wider at the top, so the gauge drops more slowly at the top and gets quicker on the way down.  Don't take too much notice of it, it's not perfect, hence checking MPG manually by brimming to the click.  Even that's not perfect though, can get air bubbles in the filler neck that fool the pump to cut off early which make you think you've used less.  There is no absolute perfect way to measure MPG unfortunately.

 

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7 hours ago, TomsFocus said:

The tank is wider at the top, so the gauge drops more slowly at the top and gets quicker on the way down.  Don't take too much notice of it, it's not perfect, hence checking MPG manually by brimming to the click.  Even that's not perfect though, can get air bubbles in the filler neck that fool the pump to cut off early which make you think you've used less.  There is no absolute perfect way to measure MPG unfortunately.

 

still makes no sense

 

fuel at half way now, usually takes another 200 miles from here until low fuel light comes on. this will mean my car has only covered 430 miles = 44 mpg including the 180 miles motorway journey at 60 mph, which is only 5 mpg more than what is should get urban and from mainly urban full tank.

Had I refilled after the 180 miles motorway trip it would have only cost £13 = 62 mpg, if I wait to refill from fuel light it tells me my mpg from full tank is just 44mpg

 

how can , if it can, this be explained ?

 

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But you didn't fill up after the motorway trip so can't say what it actually was?  The needle really isn't all that accurate, definitely not accurate enough to measure MPG from. 

The type of driving you do will also change the MPG, sitting at a constant 60mph for 180 miles should be more economical than doing 200 miles round town, so it's almost impossible to predict how many miles you've got left.

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30 minutes ago, TomsFocus said:

But you didn't fill up after the motorway trip so can't say what it actually was?  The needle really isn't all that accurate, definitely not accurate enough to measure MPG from. 

The type of driving you do will also change the MPG, sitting at a constant 60mph for 180 miles should be more economical than doing 200 miles round town, so it's almost impossible to predict how many miles you've got left.

I decided to fill my tank to the brim, only topped up £15.25 and it was at the very top , almost spilling out.

After driving the 180 miles the fuel gauge was still above where it was before topping up the £15.25, and I have calculated in the past how many litres it takes to fill from where it was at and it is 13 litres.

so if I put £13 in, it would have totally filled it to the brim again so that is 62 mpg, however if I wait until the fuel light comes on which will usually be another 200 miles from now ( calculated it many times after filling from here) it will show my total mpg from full tank to be around 44 mpg, so I find it odd, the first 180 miles saw me getting 62 mpg but falls to 44 afrer light comes on

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It really doesn't work like that.  You can't work off the gauge because it's not accurate, £15 fill won't be at the same place on the gauge every time.  Equally, the red light won't come on at the same time, so you can't possibly say how many more miles you've got, even the range on the trip computer can't work it out that well.

But forgetting that, driving at a constant 60mph on a long journey will be more economical than driving round town or driving at 80 on the motorway.

Did you also fill up and then immediately do that long trip?  The engine warming up is what most reduces MPG, so starting with a hot engine will make it better as well.  Compared to doing 180 miles in 5 mile bursts where the engine never gets warm.

 

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33 minutes ago, TomsFocus said:

It really doesn't work like that.  You can't work off the gauge because it's not accurate, £15 fill won't be at the same place on the gauge every time.  Equally, the red light won't come on at the same time, so you can't possibly say how many more miles you've got, even the range on the trip computer can't work it out that well.

But forgetting that, driving at a constant 60mph on a long journey will be more economical than driving round town or driving at 80 on the motorway.

Did you also fill up and then immediately do that long trip?  The engine warming up is what most reduces MPG, so starting with a hot engine will make it better as well.  Compared to doing 180 miles in 5 mile bursts where the engine never gets warm.

 

Yes, it does work like that. 

I have refilled from where the gauge was at to the brim several times, and it has always been 13 litres, maybe 14 litres one time.

 

Yes, I filled up and went immediately on the motorway, however my fuel is almost at half way now after just 230 miles and is where it would usually be if it were just urban driving

However the first 180 miles from the motorway only took £13, the other 50 were urban and caused the drop from 62 mpg to 42mpg.

 

Had I refilled after 180 miles it would have cost only £13 and I would have gotten 62 mpg

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15 minutes ago, Fastlife91 said:

Yes, it does work like that. 

I have refilled from where the gauge was at to the brim several times, and it has always been 13 litres, maybe 14 litres one time.

 

Yes, I filled up and went immediately on the motorway, however my fuel is almost at half way now after just 230 miles and is where it would usually be if it were just urban driving

However the first 180 miles from the motorway only took £13, the other 50 were urban and caused the drop from 62 mpg to 42mpg.

 

Had I refilled after 180 miles it would have cost only £13 and I would have gotten 62 mpg

Look, what you'd be better doing is just fill the car to the brim, reset the trip meter, drive it until the light comes on and then do an MPG calculation over the whole tank.  Without a crystal ball nobody can say if/why it would suddenly get less MPG at a certain point.

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We'll have to agree to disagree on the fuel gauge I think lol.

I think I'm missing the point here now though?  You did an economical journey at an estimated 62mpg...then did uneconomical journeys and overall it has dropped, I don't see why that's unexpected.  Unless you're saying it's too much of a drop?  I really can't say how much you'll be doing round town so don't know. 

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6 minutes ago, TomsFocus said:

We'll have to agree to disagree on the fuel gauge I think lol.

I think I'm missing the point here now though?  You did an economical journey at an estimated 62mpg...then did uneconomical journeys and overall it has dropped, I don't see why that's unexpected.  Unless you're saying it's too much of a drop?  I really can't say how much you'll be doing round town so don't know. 

Hello

I drove to Llandudno and back from Skelmersdale, was 180 return mile journey. Fuel gauge showed still had more diesel in than before I put £15 in. So I could see that had I put £13 in, my tank would be full to the brim again which = 62 mpg

 

Then I come off motorway and over 2 days I drove a further 50 miles around town, which by then I was at half fuel tank after covering 230. So it takes 22 litres from half to brim, so calculate 230 miles from 22 litres and it then shows my mpg had dropped from 62 mpg to 44mpg, just from driving 50 miles urban.

 

 

 

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16 minutes ago, jmurray01 said:

Look, what you'd be better doing is just fill the car to the brim, reset the trip meter, drive it until the light comes on and then do an MPG calculation over the whole tank.  Without a crystal ball nobody can say if/why it would suddenly get less MPG at a certain point.

See above

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That sounds about right, your average urban MPG could easily be in the 30s in stop start traffic on a cold engine.

Has the MAF wiring been sorted yet?  As that'll have more of an impact on the urban MPG than cruising MPG as well.

 

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8 minutes ago, TomsFocus said:

That sounds about right, your average urban MPG could easily be in the 30s in stop start traffic on a cold engine.

Has the MAF wiring been sorted yet?  As that'll have more of an impact on the urban MPG than cruising MPG as well.

 

Hi

 

 

Well I'm usually getting 38 to 40 mpg around town, if when my fuel light comes on I have covered only 430 miles, that will be 44 mpg, only 4mpg than urban. the written mpg for my car

urban 39.2, extra urban, 64.2, combined 52.3,

But from the first 180 miles I used only took 13 litres <£13, had I filled up from then I would have got 62 mpg

I got a call from a car breakers yard in London last week, said they had 6 wire loom from same car type, a 1.8 TDCI with same engine code, KKDA (not sure if that is same code as can't find mine)

Instead of giving me whole loom he agreed to just send me the plug with a few inches of wire and charged £50

Got wires the other day but not all the 6 wires are same color as I have on my loom. somebody said it doesn't matter so much about that, so I will go take it to an auto electrician and have them wire it up, hopefully will not take long.

 

 

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The wires colour won't matter as long as they go to the right places.  KKDA is the code for the 1.8TDCI between 2004 and 2011 so that should be fine. :smile:

40mpg round town is pretty good, better than my 1.6 gets if I don't do any extra urban (motorway/B road).  I understand what you're saying that it would've been 62mpg for the first quarter, but I don't think anyone can explain the drop other than the more miles you do the more accurate the average will become. 

Maybe if you keep filling up at the first quarter you'll always have better MPG? :wink:

 

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57 minutes ago, TomsFocus said:

The wires colour won't matter as long as they go to the right places.  KKDA is the code for the 1.8TDCI between 2004 and 2011 so that should be fine. :smile:

40mpg round town is pretty good, better than my 1.6 gets if I don't do any extra urban (motorway/B road).  I understand what you're saying that it would've been 62mpg for the first quarter, but I don't think anyone can explain the drop other than the more miles you do the more accurate the average will become. 

Maybe if you keep filling up at the first quarter you'll always have better MPG? :wink:

 

I thought they had color s though, so that you know they go in the right places, if the colors don't matter then how do you know which one goes where?

 

I could keep filling from first quarter but means going back petrol station every day

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It takes a bit more time, but you just use a wiring diagram to see where each colour goes and match it to the pins in the plug.  For example, if pin 6 is an earth, as long as it goes to the earth on the cars loom it doesn't matter what the colours are.

On old French cars they use numbers instead of colours...problem is the numbers wear off over time!  Can make for some interesting wiring on those lol!

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1 hour ago, TomsFocus said:

It takes a bit more time, but you just use a wiring diagram to see where each colour goes and match it to the pins in the plug.  For example, if pin 6 is an earth, as long as it goes to the earth on the cars loom it doesn't matter what the colours are.

On old French cars they use numbers instead of colours...problem is the numbers wear off over time!  Can make for some interesting wiring on those lol!

Right, okay.

So where the wires have been cut on my car, he can attach say a green wire onto the existing blue wire e.t.c. ?

 

Also am unsure if on the mk2 2006 Focus Sport 1.8 TDCI, should it be a 6 pin or 4 pin sensor ?

 

It had a 4 pin sensor on the car when I bought it but had 6 wires connected to it.

then when Brian took the 6 pin sensor off and put a 4 pin sensor on he put 2 wires in a loop, he later cut off the 2 wires he had looped.

Now I have a 6 pin and a 4 pin sensor, I think it'd be best to put the 6 pin sensor on and I have the new plug now which has 6 wires on it, so the auto electrician can wire the 6 pin sensor on with 6 wires attached to it

any ideas ?

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Yes, as long as the wires go to the right place he can attach a green to a blue.

Well if the car loom has 6 wires on it you need the 6 wire MAF for the ECU to get the correct readings so yeah, fit the 6 pin one. :smile:

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4 minutes ago, TomsFocus said:

Yes, as long as the wires go to the right place he can attach a green to a blue.

Well if the car loom has 6 wires on it you need the 6 wire MAF for the ECU to get the correct readings so yeah, fit the 6 pin one. :smile:

Phew !

well it seems that this MAF nightmare is almost over.

The auto electrician shop in mind charges £25 per hour but said he has to do his diagnostic test to start with which costs £25, and  £25 an hour onwards, how long do you think it would take approximately for him to complete the job ?

It gives me an idea if when I can take it in to get done as I need to know how much money it will cost on average, I can work this out at an approximate guess by how many hours it will take.

 

Will my car increase on mpg and power once  MAF is fixed ?

The same day afrter the wires were cut , when you accelerate hard I hear a tiny thud from engine and it refuses to accelerate or accelerates very slow, somebody told me because ECU is confused by bad wiring as it only happened hour after wires were cut

 

 

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Just to solder 6 wires together won't take more than an hour, fitting the sensor only takes a few minutes.  You'll probably have to pay for the full hour even if it only take 45 minutes though.

Yes you should notice an increase in power immediately.  It may take a few days before the MPG improves though.  It sound like your lack of acceleration is caused by the MAF wires, but we can't say for sure until that is fixed.  If there is another problem, the diagnostic test should show that though.

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1 hour ago, TomsFocus said:

Just to solder 6 wires together won't take more than an hour, fitting the sensor only takes a few minutes.  You'll probably have to pay for the full hour even if it only take 45 minutes though.

Yes you should notice an increase in power immediately.  It may take a few days before the MPG improves though.  It sound like your lack of acceleration is caused by the MAF wires, but we can't say for sure until that is fixed.  If there is another problem, the diagnostic test should show that though.

Hi

 

thank you, hopefully it should not take them too long and I hope they will be able to find the wiring diagram for it without any problems.

It was about 1 hour after he cut the wirers, I was on the motorway and doing around 65 mph, went for an overtake and once got around 70 I hear a very faint thud and then my car felt sluggish, would not accelerate or would be very slow to accelerate , happens when accelerating hard at any speed and going fast uphills.

but when I temporarily reset the code it does not have this issue until light comes back on

Also when car is very cold and you first get in and press the gas, feels like car will stall as RPM'S drop so you need to take feet off gas and wait a few seconds for it to rev properly, again only happened after wires cut.

 

 

Would be good to have improved power and mpg.

 

 

Can you please answer my other thread regarding coolant expansion tank issue ?

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