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Trip computer, really far out - MPG |
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simonmcc
 
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  Member No: #26288
Joined: Jun 24 2012Location: Coleraine, NI |
Wow guys, a lot of replies, sorry, I haven't been on the forum since. To answer the question, the car was serviced just before I got it, about 4 weeks ago now. It is running what I believe to be the standard 16" wheels with 205/55 R16 tyres (IIRC). I reset the computer when I bought the car, I have now filled it 3 times, the last fill measured 53MPG, and the trip computer, which now has about 2000 miles on it is reading 64.1 MPG. My suspicion is that it is inaccurate at recording long downhill sections, where it goes to 999MPG, I had an Astra years ago that seemed to skew the figures based on how much driving I did with my foot completely off the accelerator. Incidently, I use the cruise control a lot, and it has flashed up a message several times saying that the speed monitor is faulty (can't remember the exact wording). It then repeats the message anytime you press the controls. However, the limiter still works, and it goes back to normal operation if you turn the car off and on again. I haven't checked the accuracy of the speedometer yet. |
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PentlandC4
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   Member No: #13613
Joined: May 08 2010Location: Deepest Wiltshire |
The 999mpg figure is the car literally using no fuel - ie coasting in gear and no need to consume fuel. It's been pretty well documented on the site that cruise control is bad for mpg, whereas the speed limiter has a far more positive effect. As a comparison, the speedo on my SX generally reads 1mph below my TomTom, checked on clear, straight stretches with few trees etc. As a further comparison, if I match the speed of an artic on the motorway, my C4 will read 55/56mph versus their 56mph. However, the mpg reading for the last tank was 18% out over actual fuel used, so I'll check it over the next two or three tanks. As an aside on speedo accuracy, we have a long 40mph motorway stretch on the M9 with average speed cameras, and my C4 is, apart from lorries, the only car in the outside lane. |
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PentlandC4
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Member No: #13613
Joined: May 08 2010Location: Deepest Wiltshire |
I've recorded my fuel economy over the last three tanks and 2109 miles. Fuel economy has been 60mpg over the distance versus 72mpg on the trip computer over that period, so the trip computer over-reading by approximately 17%. |
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BigJohnD
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  Member No: #82
Joined: Jan 22 2007Location: Hoylake |
PentlandC4 wrote ... the trip computer over-reading by approximately 17%. That's way out! Can you check your speedo against a SatNav? And is your clock accurate? |
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PentlandC4
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Member No: #13613
Joined: May 08 2010Location: Deepest Wiltshire |
Yes, see post above - speedo is very accurate! It sits a maximum 1mph below my TomTom on a clear stretch of motorway, and pretty much matches lorries sitting at 56mph. I wonder if it's an SX thing, as simonmcc's computer is similarly inaccurate? | ||
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BigJohnD
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Member No: #82
Joined: Jan 22 2007Location: Hoylake |
I've just filled my tank, it took 56.8 litres. The trip reading was 738 miles. I calculate that to be 59.04 mpg. The trip computer was showing 60.0mpg. Not a lot of difference IMO. What does concern me is that the odo probably over-reads, that is, I had driven less than the recorded 738 miles. My reason for thinking this is that the speedo is always reads 2 to 3 mph higher than the SatNav, i.e. an indicated 70mph is an actual 68mph; an indicated 30mph is an actual 28mph. I haven't yet figured out a correction factor, or indeed a way to accurately measure distance travelled. The clock is more or less accurate, losing less than a minute in a couple of months, as measured by the BBC time signal. |
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routemaster1
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 Member No: #574
Joined: Jul 08 2007Location: Dorset |
As I have posted before, we have 3 cars with trip computers. The C4GP is a little optimistic, and our C3 and Clio both are wildly optimistic by about 10%. The speedos are all accurate enough by GPS. The Clio indicates use of 10 gallons of fuel, but actually takes 11 gallons to fill. The indication from this is that the fuel input into the computer calculations is likely to be the issue. Just don't take any readings too seriously; the most accurate way is by filling and calculating yourself. Even this is frought with issues as it is just about impossible to fill to the same level each time. | ||
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BigJohnD
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Member No: #82
Joined: Jan 22 2007Location: Hoylake |
routemaster1 wrote ... The indication from this is that the fuel input into the computer calculations is likely to be the issue. Just don't take any readings too seriously; the most accurate way is by filling and calculating yourself. Even this is frought with issues as it is just about impossible to fill to the same level each time. I was trying to point out above that odo/mileage is just as likely to be inaccurate. Re-fuelling your car at the same pump and letting the first auto cut-off be the full point is about as good as it gets. |
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RichieW
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Member No: #58601
Joined: Aug 08 2021Location: Norfolk |
Love my c4 1.6 hdi, it's very economical. That pesky trip computer annoys me though. Did a brim to brim of 69.8 this week. Computer says 76.3!! Yes I hypermile religiously (when safe to do so). My commute includes no dual carriageway but is mainly A roads, only hit 2 towns which I have to cut through. |
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