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djnunny

Info needed on MPG

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Hello everyone,

Ok so on Saturday I did a nice long journey went from Leeds area to Newcastle and back around 200 mile trip.

My trip computer avg was sitting at 65mpg which is quite nice if its true am not so sure. I used half a tank which if parkers is right on a 53 litre tank I used 26 litres.

Using a mpg calculator that’s working out I did about 34mpg which I think is rubbish on a straight run it was 1 road no stops steady 65mph.

Is there something I should be looking at on car it’s recently had fuel filter done (what a bugger to get going again they are!!) new air filter about month ago and Oil & filter changed bout month ago also. one thing I should add is last time I let the care get below Qtr. tank it cut out and wouldn't get going again I had to get someone out to drag fuel back up from the tank to the pump so I don't know if there’s a leak somewhere wouldn't know how to find it if there was.

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You cannot work out an accurate consumption if you're looking at the fuel gauge position. Brim the tank, do your trip, brim the tank again and use that fill quantity for calculating consumption accurately.

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4 minutes ago, YOG said:

You cannot work out an accurate consumption if you're looking at the fuel gauge position. Brim the tank, do your trip, brim the tank again and use that fill quantity for calculating consumption accurately.

This, plus the fuel gauge does not include the 5L reserve, so at best it will show 48 L from full to zero.

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i see well i cannot do this until the weekend as work isnt ideal to work this out only 20 mile trip,

ill have a run on the weekend to get a good judge, just still seems to be alot to me even if the gauge isnt being as accurate as it should.

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It may be using a lot, but brim to brim will give you an accurate figure to start from.

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12 hours ago, djnunny said:

around 200 mile trip.

My trip computer avg was sitting at 65mpg which is quite nice if its true am not so sure. I used half a tank

The tank gauges are pretty rubbish, on my car 1/2 full on the gauge is less than 20l used from full. I almost always get about 200 miles to the 1/2 full point, then the gauge drops more slowly, and it usually works out at about 50mpg from fill to fill.

I always use the brim fill method, and since I do some business mileage in my car, I have recorded every fill and mpg since I bought it, nearly 6 years ago!

65 mpg sounds a bit optimistic for a 1.8, unless there is a lot of decent motorway runs. In Winter I usually get about 47 to 49 mpg, and in summer it goes up a bit, like 49 to 53 mpg. That is a mix of use, but usually no motorway, not much dual carriageway, and some towing. So I reckon it is not too bad.

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thanks for the replies guys, am going to brim it on the next fill i usually do around 200 miles in the week which going of the gauge i use around half tank for that aswell thats mixed roads for work and little pottering around town ill see what its doing then i suppose.

 

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I use an app :D

Road Trip

 

Works perfectly. You can play with all the settings and get all the info you need as long as you yourself input the correct figures every time you fill your car :D

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5 hours ago, JamesMalin said:

I use an app :D

Road Trip

 

Works perfectly. You can play with all the settings and get all the info you need as long as you yourself input the correct figures every time you fill your car :D

looks brilliant although i'm using android ill try and find an alternative should be one :) thanks for that

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ok so did another run today did 93 miles and brimmed the tank,

again running steady no problems needle said qtr tank used so i brimmed it again once i got back and it filled 8.6 litres with mpg calculator thats about 49mpg so am pretty pleased with that

 

so what is the problem with needle's they are not reliable at all

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16 minutes ago, djnunny said:

93 miles and brimmed the tank,

again running steady no problems needle said qtr tank used

The fuel gauge & sender system is probably best termed cheap and nasty.

90 to 100 miles is just what I get on 1/4 tank used. I am now just below 1/2 full, and have driven about 250 miles.

Part of it may be the sender & pump unit itself, it takes up space at the top of the tank, reducing the fuel capacity in the top half of the tank. But why the software can not compensate for the non-linearity, I do not know. I guess the designers either could not be bothered, or ran out of time. Everything is done (or half done, usually), in a tearing hurry these days.

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My trip comp average sits at 99.9 all the time unless I reset it, in which case it climbs quite quickly up again.  Instant mpg only dips when accelerating but if holding steady speed in any gear (obviously a speed suited to that gear) then instant read 99.9 mpg.  My fuel light goes on when the needle hits about 1/8th tank but the trip says 250 miles to go =]

 

The car is still under 3 month warranty from Evans Halshaw so I'm thinking of taking it back to them to sort it but don;t know if they'll do anything about it.

sorry for the hijack - not intended =]

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6 hours ago, Phil21185 said:

My trip comp average sits at 99.9 all the time unless I reset it, in which case it climbs quite quickly up again.  Instant mpg only dips when accelerating but if holding steady speed in any gear (obviously a speed suited to that gear) then instant read 99.9 mpg.

The in-car mpg calculations work (almost certainly) from the injector pulse width and fuel pressure. The ECU tries to keep track of the amount of fuel being delivered to the engine on each stroke like this. But it all depends on various calibration factors, as each batch & type of injector will be different. Often this mpg figure starts out rather inaccurate, then improves a bit over a few tank fills. It can use the tank gauge to measure the large change of a fill, and knowing the total mileage done, it can adjust the injector calibration factors to match.

The tank gauge on its own would be far too coarse to use for the mpg on a minute by minute basis.

Your 99.9 sounds like it is either a software error, ask them to check for updates, though they should do this anyway. Or the calibration factors have gone badly wrong for some reason, and need to be reset. An ECU reset all adaptions, or similar procedure, might do it. The dealer's system should be able to do this if needed.

But before going to the dealer, it is worth comparing the tank gauge readings before and after a decent sized fuel fill-up, against the fuel station pump. If the tank gauge is badly out, then that could be why the mpg calibration has gone wrong.

Also frequent small top-ups of fuel may make it inaccurate, as said above the tank gauge performs rather badly over some parts of its range, but is better when used over a large change from near empty to full.

 

 

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Thanks for this. After a near empty tank gets £35 (around 30 litres) I get around 3/4 of a tank but that quickly drops and settles to about 5/8ths. I think I am right in saying that diesel froths up when going in the tank so that may account for that? Going by most folk saying mk3 tanks are 50 litres or so this seems about right.

Dash will tell me I have 700+ miles in the tank...

I can lose over 150 miles from the dash simply by overtaking someone but the avg mpg barely alters at all. 

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On 6/21/2017 at 11:41 AM, JamesMalin said:

I use an app :D

Road Trip

 

Works perfectly. You can play with all the settings and get all the info you need as long as you yourself input the correct figures every time you fill your car :D

I just came here to say the exact same thing.  I've used Road Trip since the guy ran beta tests.  It's a superb app, cannot recommend it enough.  If you're geeky enough to fill it in religiously, it gives great stats!

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7 hours ago, vibeone said:

I just came here to say the exact same thing.  I've used Road Trip since the guy ran beta tests.  It's a superb app, cannot recommend it enough.  If you're geeky enough to fill it in religiously, it gives great stats!

I have used it from day one it appeared on the itunes :D

 

Great app. I even input all other service parts/intervals etc.

 

Have done for my past 4 cars :D

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8 hours ago, vibeone said:

If you're geeky enough to fill it in...

Ohhh, I'm geeky enough alright! Just ask my wife =D

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16 hours ago, Phil21185 said:

Ohhh, I'm geeky enough alright! Just ask my wife =D

I'm with you.  My last car I had 4 and a half years worth of fill ups including fuel brand :S

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well i couldnt use that one as not using any apple products what so ever, but found a android alternative i have started to use and will be filling it out regularly as i fill up to keep an eye on usage. (edit) sorry forgot to put app name lol its on the play store as "Fuel Log"

as for the 99.9 mine does this which is why i stared watching it in the first place! wasn't really keeping an eye on it til notice it doing this if i am on the motorway keeping steady it jumps straight to 99.9 but if i put a bit of pressure on the pedal it quickly drops down around 30-40

be interested to see if this is software that needs updating heard a few people before saying they needed it.

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