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Citroen and their lying trip computers!!!
jps - 5/9/13 at 08:02 AM

Probably should not be surprised at all but still am...

There is a handy trip computer on the daily driver - Citroen C3 1.4 Hdi (not my choice - but hey-ho at least it is mega-economical I THOUGHT....) which told me since the last fill-up I had squeezed out 72.2 mpg with my frugal driving.

However the - not so scientific - 'fill to the brim, count the miles, fill to the brim again, work out the mpg' approach suggests I got 59mpg.

Bit of a difference there! Are the trip computers just nonsense - or is it just Shitroen who are flattering with their figures?

MikeRJ - 5/9/13 at 08:15 AM

You can only ever get an accurate figure by brimming the tank and recording the mileage. The trip computers are estimates at best, though some are much better than others.

luke2152 - 5/9/13 at 08:21 AM

When I fill my a6 up it tells me I wont have to fill up for 1600km. This is wildly opimistic and 800 would be normal. 1000 would be good

AndyW - 5/9/13 at 08:30 AM

Mine is magic. The car still drives when the trip computer says no fuel, then I stop, and when I start I have 10 miles of fuel left. I have never tried, but if I keep stopping maybe the tank will refill completely. Unless the computer tells me otherwise.

I never take what they say as fact, more of a guessstimate

Andy

steve m - 5/9/13 at 08:44 AM

I like to play the game. of get to a destination with more fuel in the tank, than when I left home

If fill to the brim, it shows 760 miles to empty, but on an 80 miles journey recently all motorway, I had left in the tank

828 miles,

very odd !

eddie99 - 5/9/13 at 08:52 AM

My Alfa is very optimistic as well... Normally says about 5more mpg averaged than i actually get... Oh and i have managed the range of what it first said on a fill up, but that was trying with extreme care and as economical as possible.

PSpirine - 5/9/13 at 10:50 AM

You'll find 5-10% is the general optimism expressed by most OEMs on the trip computer. Some are worse than others (as in, the error is built in), but I shan't say which are good and which are bad.

Keep in mind that 7mpg error on 70mpg is the same percentage as 2mpg error on a 20mpg indicated. A lot of people seem to be catching onto the trip computer errors recently because the overall mpg has increased so much so the absolute error seems worse, whereas previously someone who was getting 20mpg instead of 22mpg wouldn't really have thought much of it.

Brim to brim is the way to go. Plenty of very handy iPhone/Android apps to help you track it..

MikeR - 5/9/13 at 11:44 AM

My SMAX is about 10% out - has been consistently. Frustrates me as I'm sure they should be able to calculate it better.

whitestu - 5/9/13 at 12:02 PM

My Mondeo is about 10% out as well. My old Passatt was the same but my Prius was spot on. Without checking you would think both the Passat and the mondeo were more economical than the Prius but in reality the opposite was true. I'm sure the marketing departments have the final say in accuracy of these things.

skov - 5/9/13 at 12:08 PM

quote:
Originally posted by PSpirine
Brim to brim is the way to go. Plenty of very handy iPhone/Android apps to help you track it..


Ah, but that assumes the odometer is spot on. I wouldn't be surprised if that was a few percent out on many cars too!

jossey - 5/9/13 at 12:09 PM

My 1 series is pretty good.

One thing to note is when you drive your car 100 miles by the car speedo there is a chance you have done 95 miles.

That said my car says I average 62mpg as you said I did the test.

580 miles later using gps my car said i had only done 560 but I worked out that it did 60.5 mpg

The old 5 series made up the figures it was always 10mpg out lol

nick205 - 5/9/13 at 12:12 PM

They're only calculating the fuel used, not actually measuring it. This will always be inaccurate to some degree although the OP's sounds pretty poor to me. The last few VAG cars I've run have all been optimistic by 5-15%.

TBH you should know without a computer how efficiently or not you're driving

PSpirine - 5/9/13 at 12:23 PM

quote:
Originally posted by skov
quote:
Originally posted by PSpirine
Brim to brim is the way to go. Plenty of very handy iPhone/Android apps to help you track it..


Ah, but that assumes the odometer is spot on. I wouldn't be surprised if that was a few percent out on many cars too!



Speedo on modern cars is usually good (assuming no weird tyres fitted) - within <1% of GPS verified.

In any case, the apps allow you to put in correction for speedo/odo readings.


Once again.. the trip computer "error" is not through miscalculation, it is a deliberate factor applied by many OEM's, normally around 5-8%, although the Germans seem to be the cheekiest and get away with closer to 10%.

whitestu - 5/9/13 at 02:39 PM

quote:

Speedo on modern cars is usually good (assuming no weird tyres fitted) - within <1% of GPS verified.


That's not my experience. Most seem to over read by 5-10%, with the exception of Citroens whihc always seem to read very accurately!

[Edited on 5/9/13 by whitestu]

britishtrident - 5/9/13 at 05:23 PM

quote:
Originally posted by steve m
I like to play the game. of get to a destination with more fuel in the tank, than when I left home

If fill to the brim, it shows 760 miles to empty, but on an 80 miles journey recently all motorway, I had left in the tank

828 miles,

very odd !


Range is worked out on the current average MPG, on a motorway average MPG is always going to be higher than the average normal suburban/urban running mpg.

[Edited on 5/9/13 by britishtrident]

britishtrident - 5/9/13 at 05:35 PM

On a lot of modern cars the speedo is calibrated to read about 4% to 5% fast but the speed via OBD II is usually very accurate if compared to GPS.

Peugeot got into a lot of trouble about 10 years because there odometers were reading 25% to 30% high.


On Rovers the BMW trip computer MPG is usually pretty accurate in normal suburban use but is more optimistic in motorway conditions.

turbodisplay - 5/9/13 at 05:39 PM

I had an indicated 63 77mpg on a pug 407. Actual figure 49 -53!