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Trip computer, really far out - MPG

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simonmcc   
Thu Jul 12 2012, 05:14am
Member No: #26288
Joined: Jun 24 2012
Location: Coleraine, NI
I've had my 2005 C4 1.6 HDI for a couple of weeks now, done about 1300 miles, and trip computer is showing 62.7MPG, and when I work it out over my two fills they were 53/54 MPG.

I knew there was the possibility of it being out a bit, but that's a really surprising amount of error! (~ 16%)
Biohead   
Thu Jul 12 2012, 05:24am

Member No: #6049
Joined: Nov 05 2008
Location: Bracknell
Something somewhere suggests that's not right, as the C4 is usually pretty much on the mark with MPG figures.

When was the last time it was serviced?
PPP7   
Thu Jul 12 2012, 06:23am
Member No: #135
Joined: Feb 25 2007
Location: South Midlands
Maybe you have non-standard wheels/tyres fitted?

Are you looking at average or instant MPG?
BigJohnD   
Thu Jul 12 2012, 10:30am

Member No: #82
Joined: Jan 22 2007
Location: Hoylake
Maybe you need to reset the trip computer, though it's usually accurate to a few %.

Does your car's clock lose/gain a lot of time? And as said before, do you have standard tyres?
routemaster1   
Thu Jul 12 2012, 12:31pm
Member No: #574
Joined: Jul 08 2007
Location: Dorset
I have 3 cars with computers. The C4Picasso is accurate to ~3mpg. But our C3 and Clio both claim mid to high 60s, but actually only give 58 to 60 mpg. So I don't think your readings are unusual.
PentlandC4   
Thu Jul 12 2012, 03:15pm
Member No: #13613
Joined: May 08 2010
Location: Deepest Wiltshire
simonmcc wrote ...

I've had my 2005 C4 1.6 HDI for a couple of weeks now, done about 1300 miles, and trip computer is showing 62.7MPG, and when I work it out over my two fills they were 53/54 MPG.

I knew there was the possibility of it being out a bit, but that's a really surprising amount of error! (~ 16%)


Mine is similarly inaccurate - 2005 HDi 92 SX - on standard tyres, pressures correct etc.
PPP7   
Thu Jul 12 2012, 05:48pm
Member No: #135
Joined: Feb 25 2007
Location: South Midlands
Maybe Citroen only calibrate the electronics for 17inch wheels and 205/50 tyres? As my C4 computer is within 1-MPG of a manual calculator check after filling the tank, and proven several times.
routemaster1   
Fri Jul 13 2012, 12:48am
Member No: #574
Joined: Jul 08 2007
Location: Dorset
Hamilton wrote ...

Maybe Citroen only calibrate the electronics for 17inch wheels and 205/50 tyres?


I doubt this, as the rolling radius of all sizes of tyres should be virtually the same. I can tell you that on our Clio, when it tells me I have used 10 gallons of fuel, it actually requires 11 gallons to fill, so it is the fuel metering which is at fault.
Biohead   
Fri Jul 13 2012, 03:12am

Member No: #6049
Joined: Nov 05 2008
Location: Bracknell
Routemaster: Do you have a GPS unit of some sort to compare the speedo speeds to actual speeds? If it is a wheel calibration fault, then there should be some pretty noticeable discrepancy.

If it's not, I'd expect the speedo to be within 1-2mph.
VTRPLUS27   
Fri Jul 13 2012, 04:07am
Member No: #24370
Joined: Feb 20 2012
Location: Hants
my sat NAv speed is +3.5 compared to my speedo at 60 to 70 mph
At 30 on sat nave speedo is 29.
routemaster1   
Fri Jul 13 2012, 12:51pm
Member No: #574
Joined: Jul 08 2007
Location: Dorset
Biohead wrote ...

Routemaster: Do you have a GPS unit of some sort to compare the speedo speeds to actual speeds? If it is a wheel calibration fault, then there should be some pretty noticeable discrepancy.

If it's not, I'd expect the speedo to be within 1-2mph.


Speedos are all fine. But it isn't the speed that is the issue. It is distance divided by fuel used. As I said, the Clio indicates use of 10 gallons of fuel, against a refill of 11 gallons. Do the sums and it's about 10% out. The C3 doesn't tell me how much fuel I've used.
FakeConcern   
Fri Jul 13 2012, 02:00pm
Member No: #24127
Joined: Feb 05 2012
Location: Bexhill on Sea
When I worked out several brim to brim av MPG mine is about 2MPG out.
Biohead   
Fri Jul 13 2012, 02:46pm

Member No: #6049
Joined: Nov 05 2008
Location: Bracknell
routemaster1 wrote ...

Speedos are all fine. But it isn't the speed that is the issue. It is distance divided by fuel used. As I said, the Clio indicates use of 10 gallons of fuel, against a refill of 11 gallons. Do the sums and it's about 10% out. The C3 doesn't tell me how much fuel I've used.


Just wondered, as the C4 calculates how much fuel it's used electrically - it's narrowing down the fact that perceptual speed doesn't play a part in the calculation. It must be purely engine loading.
ror100   
Fri Jul 13 2012, 02:49pm
Based on a years motoring in my C4 my computer is always 3% to 5% out. Part of this is to do with the conversions factors, as the metering is measured using litres / km, and then converted to mpg for us UK folk.

Either way, a 60 mpg readout from my computer always calculates to 57mpg in real life, as recorded by Fully.com in my sig, and also on my own independent spreadsheets.

This is a long shot, but are you calculating using UK Gallons (1 gallon is 4.54609 litres) and not US Gallons (1 gallon is 3.78541 litres), this would possibly account for the difference.
PentlandC4   
Sat Jul 14 2012, 10:45am
Member No: #13613
Joined: May 08 2010
Location: Deepest Wiltshire
ror100 wrote ...

This is a long shot, but are you calculating using UK Gallons (1 gallon is 4.54609 litres) and not US Gallons (1 gallon is 3.78541 litres), this would possibly account for the difference.


I did double check... UK gallons!
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