Trip Computer Accuracy

[KeithC5]

I have a 2005 1.6HDi VTR, I reset the trip computer with every fill up (to the brim) the computer is always stating that I am averaging around 52-54 mpg, however when i fill the tank and do the maths, it is actually doing around 43-45 mpg, does anyone else have this problem? Any free fixes?

[72dudes]

I have a 2005 1.6HDi VTR, I reset the trip computer with every fill up (to the brim) the computer is always stating that I am averaging around 52-54 mpg, however when i fill the tank and do the maths, it is actually doing around 43-45 mpg, does anyone else have this problem? Any free fixes?

 

 

On my 2001 2.2 HDi the computer is about 5% optimistic (i.e. tells me it's doing 47 MPG when brim to brim it's doing 44.5 MPG). In my experience of various cars with trip computers, this is quite normal.

 

However, yours does sound as though the reading is way out (almost 20%). Sorry, don't know of any free fixes, but a new software download from a dealer or specialist may help.

 

Also, 43-45 sounds quite low for the 1.6 HDi unless you are doing a lot of short journeys. Without wishing to sound condescending, are you sure you are doing the calculation correctly? (i.e. 1 gallon = 4.546 litres)

[KeithC5]

On my 2001 2.2 HDi the computer is about 5% optimistic (i.e. tells me it's doing 47 MPG when brim to brim it's doing 44.5 MPG). In my experience of various cars with trip computers, this is quite normal.

 

However, yours does sound as though the reading is way out (almost 20%). Sorry, don't know of any free fixes, but a new software download from a dealer or specialist may help.

 

Also, 43-45 sounds quite low for the 1.6 HDi unless you are doing a lot of short journeys. Without wishing to sound condescending, are you sure you are doing the calculation correctly? (i.e. 1 gallon = 4.546 litres)

 

Thanks for the reply, I do a mixture of journeys, but mainly 35 miles round trip to/from work each day with the odd longer journey, the car has done 85,000 miles and just had the FAP filter changed. I don't drive hard at all, (maybe I should to see what happens to the mpg).

 

I do use 4.546 as the calculation (thanks for checking that).

 

It appears that after I fill up and reset the TC it goes up and down as you would expect for the first 50 or so miles then seems to settle down and not change really.

[72dudes]

It appears that after I fill up and reset the TC it goes up and down as you would expect for the first 50 or so miles then seems to settle down and not change really.

 

That sounds right - as the average is recalculated every 20 seconds (something like that, anyway), the peaks and troughs will get smoothed out. Hard acceleration or extra smooth steady speed driving will still make the average MPG change, even 400 miles after a re-set, but not hugely.

 

Sounds like a new software download is what is needed.

Just another thought - have you compared the miles covered with the the speedo tripmeter? Likewise the average speed with the speedo (drive at a steady speed and re-set the computer, after a few seconds the average speed reading should correlate to the speedometer reading. Mine reads 67 at a steady 70, which shows normal speedometer error.)

[myglaren]

I don't drive hard at all, (maybe I should to see what happens to the mpg).

Yes, you should, every so often give it a blast down the motorway and kick it around a bit.

 

Before embarking on that though, give it a dose of Forte Diesel System Cleaner - it is a bit pricey - around £13 - but seemed to work well on mine, coupled with a 600 mile round trip at occasionally inadvisable speeds :unsure:

[Ronin]

Trip computers in any vehicle are advisory functionals only. They take into account the fuel flow through the injector and MAF sensors via the ECU and also fuel tank sensor. An average figure is then projected. But there is discrepencies.

 

It will never be as accurate as actually calculating the exact mpg you get. TC's also factor in the 6.25 ltrs of fuel that are carried in the "reserve tank" If there is such a thing AKA: the orange light/ fuel warning mode.

Unfortunately there is no provision on the fuel return pipe which sends unused fuel to the the tank when no pressure is applied to the accelerator during engine breaking and coasting.

Then there is also a variance of how much fuel is in the lines, filter and pump (diesel models), a further 0.4 ltrs for petrol and 0.85 ltrs for diesels. This is something that TC's can't calculate and therefore it has to optimistically predict the resulting lack of information.

 

In reality there is one provision that many owners fail to include when calculating mpg:

 

Ambient Temperature!

Fuel is delivered in volume and even though the fuel pump in a filling station will deliver the exact given amount there is a lot of difference in petrol and diesel volumes dependant on ambient temperature.

For instance when delivering a fuel load of 50 litres. @ 70f can be 49.6ltrs and @ 50f it will be 50.2 ltrs volume. That is why it is more fuel efficient to refuel during night time when it's cooler. Bearing in mind that an extra litre of fuel can get a further 7-12 miles in the tank depending on what vehicle you drive. I have to say though that this is more often the case when you see a free pump that has been standing empty for a while as the fuel can have a chance to decrease in mass with rising temperatures. Don't worry about temperature once it is is your car. if the tank is sealed properly that is what you get and keep.

 

If you really want to be fastigous about fuel economy:

Try refueling in a slightly different manner too. Don't pull the trigger all the way down as vapours escape back to the fuel pump's breather pipe and returns to the storage tank! Gently does it and you will get slightly more fuel in the tank and less vapour evaporation.

Avoid the temptation to brim the fuel to your filler neck as fuel vapours and spillages will end up being wasted via your breather pipe. Instead just fill up to the first click of the pump cut off.

Try it and see what you get then in mpg guys. :)

 

One last thing: We have all seen these wonder fuels that are about 10p more expensive per ltr in the forecourts of filling stations. They promise better mpg and cleaner engines etc.

Avoid them like the plague!

A well looked after car that is already getting the same average mpg that a manufacturer already states will not "magically" find any extra mpg from nowhere. The detergents that they use are inferior quality and can cause poor running when reverting back to standard fuels, thus you think you need another refill of the "good stuff" and miraculously the poor running dissapears!

 

If anything try some injector cleaner off the shelf, STP, Redex to name a few(far superior detergents) every six month and that will be more than enough. :blink:

[myglaren]

Avoid the temptation to brim the fuel to your filler neck as fuel vapours and spillages will end up being wasted via your breather pipe.

.

 

That may well have been true of older cars but AFAIK modern fuel tanks are pressurised and tank vapours vented through the charcoal filter into the engine to be burned.

[KeithC5]

But I suppose what I am wanting to know is, My TC appears to be over-stating my mpg quite considerably, do other 2005 1.6Hdi VTR C5s have the same innaccuracy or is there something that could be causing mine to be out? Is there anything I can check? The mpg I expected when I bought the car was 40-45 so I am reasonably happy with that but would like it if the TC didn't tell me such big porkies!

[myglaren]

As Ronin says, it is not very accurate for many reasons.

If you have changed your wheels or even if your tyres are a slightly different size to those in the specifications, there will be an innacuracy there.

It is driven by the speedo, which is also rarely accurate.

All thes innacuracies combine, some can neautralize others while some reinforce others.

 

I have seen many comments on the innacuracy of the trip computer and again, to refer back to Ronin's advice, the only accurate measurement is to record the actual volume of fuel at each fill-up, ideally on a spreadsheet, and add the distance travelled and let the spreadsheet calculate the current and grand averages over a period of time to gain any significant date and display variations from the average.

[800jimbo]

In my experience the speedo under reads. Assuming that the same information feeds the trip counter and computer then they will be wrong also. However, since you are basing your calculations also on the trip counter reading that would not account for any difference between you and the computer, if you see what I mean.

 

I reckon my speedo is under by about 4%, so I am getting 4% better performance than what I calculate I am getting. If only that weren't the square route of 'very little' I'd be pleased.