C5 MPG reading - accurate?

Citroenmad · Unread post by **Citroenmad** » 14 Apr 2009, 17:12

C5 owners, do you often check your actual MPG? Or do you just rely on the computers output to show you what you are getting?

I have been checking mine quite often, its usually reading 6-7mpg UNDER the actual MPG. So if my car says its doing 40 its really doing about 46.

I trip both the mileage trip and the computer trip at the same time, when i fill up. Then when i fill back up i work out my actual MPG using the usual calculations and the trip mileage and see how this corresponds to what the car says i have achieved.

This reminded me of another thread, when someone said their sons Mundaneo got much better MPG than their C5. Now doing a bit of digging on the Mon-deo forum shows they have a tendency to over read by about 3-4MPG. Now given that book figures shows C5s are more economical than the mon-deo, i thought it was odd that it should do better than a C5, but that might explain it.

Anyway, i was wondering, would the Lexia or C5 appropriate program be able to adjust the cars MPG computer figures so it reads more accurate?

Im yet to work out the estates real MPG, although it seems to be a bit more accurate as its average always shows higher than my hatch does, driven by me in my usual way.

Aside from that, its MPH reading on the dials is as accurate as they get, its about 0.5 mph over reading. A swap with the slightly larger estates wheels to the hatchback at the weekend saw the car under read by about 1.5mph. 70 on the dials was 71/2mph on the satnav.

Chris.

JohnD · Unread post by **JohnD** » 14 Apr 2009, 18:27

When I first got my C5 I checked the actual miles per gallon against the computer read-out. I've done a few 1200 miles runs both towing and driving solo when I kept a note of fuel used. There was so little difference between my records and the computer that I ceased keeping a written record.

bencowell · Unread post by **bencowell** » 14 Apr 2009, 21:15

Young Mr Citroenmad,

I understand you tell the Lexia the size of the tyres so that the computer can adjust the speedo.

Your hatchback is economical becuase the engine is in as new condition because of the 6000 miles oil changes using fully synthetic oil.

My 2.2 (twin turbo 173) underreads too, averaging 2-3 mpg under-read.

My friend tells me that Astra speedos are set to underread by 10%, my bosses BMW speedo reads higher than his Tom Tom.

Speeds over-reading is good for manufacturers. You appear to do more MPG, the car feels like it reaches the speed limit faster and is therefore better.

Before buying my second C5, I compared the speedo reading against satnav. Both my present and previous C5 (sold to Citroenmad) have speedos that are as near accurate as possible.

Ben

Citroenmad · Unread post by **Citroenmad** » 14 Apr 2009, 21:53

Good to hear from you ben - Might see you on sunday for the CCC drive?

Your right, your 6000 mile servicing has really paid off, it drives like a new car. Its due its 6K service now, which i plan to do tomorrow, just need to get the fully synthetic oil for it.

My C5 feels like new, to drive, sit in and look at. However the estate, even though is in very good condition, feels used, though they have a very similar mileage.

The C5 estates speedo over reads by 2-3mph, which is nothing. Our C15 is doing an actual 70 at around 80!

It would be nice to have the MPG read a little more accurate though, but i know what its doing as i know how much it undereads by.

Chris.

corsehf · Unread post by **corsehf** » 06 May 2009, 20:14

I feel the trip computer on my C5 is the worst or unreliable i have encountered.
I have now got used to the MPG readout being under actual figures, i.e, my computer says 34.4mpg when it's actually returning 38.5mpg.
However!, the fuel range figures are awful.
Everytime i fill up, i get a range of 410 miles, no more no less? i recently drove down to London which is a trip of around 140 miles and by driving carefully, i managed to get the fuel range up to 560 miles by the time i parked up in London. On my return back to the car, when i flicked on the ignition, the range was now showing 280 miles??
Is someone syphoning out my diesel when i am not looking!!!

dave · Unread post by **dave** » 06 May 2009, 22:19

fuel computers lie........

Citroenmad · Unread post by **Citroenmad** » 07 May 2009, 16:20

Fuel computers do lie, your right.

I have been continually chekcing mine, and it is 6-7MPG under reading. So when it tells me im doing 40MPG im really doing 46-47MPG. Actually mine is staying around 42.8MPG now, over quite a lot of miles, so thats more like 48-49MPG. Thats not bad going, as its mixed driving, town and motorway use.

I have checked the Estate now, that reads about 4MPG under, so its a bit closer but not too accurate.

You would think it would be good for a cars computer to over read, that way it makes the car look better than it is. Underreading makes it look worse and if the actual MPG is not checked some might be dissapointed with their results.

Ive not taken much notice of the miles left reading, i use it as a rough guide but i rarely look at that. Seems to be about right though, with a full tank im sure its around 600miles, will check next time i fill up.

The miles left reading is worked out by the current average MPG, so if your MPG is underreading then so will the miles to empty be.

It would be nice to be able to make the computer read more accurate, maybe with lexia, but im not sure if that is possible.

Chris.