Inaccurate MPG on trip computer

[piggoy]

Hi peeps,

I've been investigating this for a while now. I'm finding that the MPG and range on my trip computer is a bit inaccurate. By filling my car up to the brim each time and by recording the milage between trips to the pumps I've been able to calculate the true MPG.

Example - After a 200 mile run on the motorway at "normal motorway speed" my trip computer says 38mpg. But I am only calculating 29mpg based on the actual amount of fuel consumed.

Another example - After filling the car to the brim and then driving 200 miles, the recorded milage increases by 200 (obviously!), but the estimated range decreases by nearly 300 miles.

Any ideas people? Are these trip computers known for being inaccurate. I was worried that my fuel tank was perhaps leaking or something but there is no evidence of any fuel loss when it's parked up.

 

[Craigus]

It is known that they overestimate, they give a rough idea, but it has to be taken with a pinch of salt

 

[Budgie]

They all do it, I think.

Range is just a guide.

When I fill mine up it says an estimated 380miles or so. 290-310 is more like it.

 

[Beef-Mc]

You are resetting it each time you fill up aren't you?

Your average MPG will effect how far the car thinks it can continue.

 

[Scott S]

I might be wrong, but I would have thought that the range would depend on the 'instant consumption', i.e. if your current (not average) mpg drops because you floor it, the car can only assume you will be continuing at this current MPG and works out the range from there.

If not then it will just depend on the average mpg which can change and will then change the range.

My MPG shown also differs from what I work out, but only by about 1-4mpg.

My cruise controll confuses me more though, I haven't had the chance to search the forum yet to see if this has been discussed, but my digital display can show 58mph but my speedo shows 60. I expect there to be a difference between my speedo and my sat nav, but from two different parts of the same car........................

 

[Scott S]

Just searched and it seems I am far from alone with the cruise control. I realise that isn't useful to this thread, but I had started writing this before I realised.

 

[piggoy]

Basically I have singled it down to an error in the indicated amount of fuel consumed, which is used by the computer to calculate the mpg and the range. How does the trip computer measure the amount of fuel consumed? Is it linked to sensors in the fuel tank or fuel pump? I'm just wondering whether there is something malfunctioning somewhere which I could perhaps fix?

 

[piggoy]

Any ideas people?