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Discussion Starter · #1 ·
I've been monitoring the differences between my trip mpg and my actual and I am getting bizzare results!!!! How can it vary so much? The car has remained unchanged since new. Just look at the last 10 of my fillups, the tyre pressures and load settings have been the same over all of these fillups too. Most of these are over 400-500 miles per tank too, fillin up with either just one white bar left or on the final red bar.

Variance between trip and actual: (figures rounded to whole numbers for ease of reading)

Actual / Trip / Variance
54 / 58 / 8%
54 / 57 / 6%
59 / 60 / 3%
49 / 54 / 11%
53 / 57 / 7%
52 / 55 / 5%
47 / 50 / 7%
52 / 54 / 3%
43 / 46 / 6%
53 / 56 / 6%

In my case on every single fillup the trip computer has shown higher mpg than actual. Although I think Reddaz had one of his go the other way.
 

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Discussion Starter · #2 ·
Found this US article about the same problem. (VW being amonst the worst culprits for it, surprise surprise!)

http://www.edmunds.com/fuel-economy/your-fuel-economy-gauge-is-fibbing.html

Some of the comments make interesting reading too, especially the one about the overreading odometer causing you to take your car in for services more frequently than necessary and potentially costing you extra charges on PCP excessive miles.

Mine overreads by 3% according to my GPS (car says I'm doing 60 when I'm actually doing 58.5mph on GPS) but I run my tyre pressures high so mine is more accurate than people who run lower tyre pressures, drop pressures down to 30psi and you'll be getting a 5% overreading. So your car would show 30,000 miles when it had actually only done 28,500 miles! A significant amount on a PCP financed vehicle.
 

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Yes I have had one that has calculated lower then the actual mpg.

My last 10:

Actual / Trip
49.8 / 51.1
51.5 / 53.3
53.35 / 50.9
46.82 / 51.1
49 / 50
53.7 / 54.9
47.5 / 48.3
52.1 / 52.4
46.9 / 47.7
54.1 / 54.5

I notice that the trip reader seems to drop suddenly sometimes, I think it resets. Not sure why it does this.
 

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Discussion Starter · #4 ·
Yours are alot more accurate than mine Raddaz. By 'drops' do you mean the instant mpg drops to something silly like 3mpg then returns back to 50mpg or whatever it was at? I think that is when it is running a regen on the cat although I may be wrong - it does it on mine once or twice on my long motorway commute. I know that it does this on our works diesel cars when it does a regen on the DPF's so maybe the petrol cats have a similar thing?
 

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mud4fun said:
Yours are alot more accurate than mine Raddaz. By 'drops' do you mean the instant mpg drops to something silly like 3mpg then returns back to 50mpg or whatever it was at? I think that is when it is running a regen on the cat although I may be wrong - it does it on mine once or twice on my long motorway commute. I know that it does this on our works diesel cars when it does a regen on the DPF's so maybe the petrol cats have a similar thing?
Surely unleaded cars are much cleaner and don't produce anything like as much soot and carbon build up as a diesel that they require a regen to "burn" it off.
 

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I don't think petrols have regens.
 

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Discussion Starter · #7 ·
Not sure what it is then, mine definitely drops to say 3mpg for about 2 or 3 seconds and then goes back to normal. It does this about once every 100 miles or so.

Just another fault to add to the list?
 
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