Trip Computer Accuracy - Full Test

[Timelord1.9Tdi]

Hello All

Ok 4,100 miles later running a full test using FULL scope of consideration to the varribles that comes with the methods people use when it comes to calculating the mpg on the Brim to Brim Method.

Here is the method I have used to calculate Brim 2 Brim fillings.

1. Go to same garage every Fill Up

2. Go to same pump (Pump 4) fill up using same nozzle.

3. Nozzle in half way allowing gap for vapour to escape.

4. Fill to the auto cut out of the pump no more no less

5. Take full data 1) Miles Done 2) Liters 3) Trip Comp MPG

6. Calculate using following formula : - Miles done / liters x 4.54609 = MPG

Then I cross referance to the trip computer claims. There are of course varribles that no one can change when occuring. After minimising those varribles that can be standardised.

My findings are that the trip computer to brim2brim differance is 99.7% accurate which is a average figure over the 4,100 miles. Which can be attributed to the varribles of temperature and viscousity of the fuel etc etc

In Conclusion I would confidently now state that what the computer say's is actually accurate...... which is a supprise as I always considered trip computers a joke. But hey a methodical approach to finding the truth has revealed the fact that they are accurate with n acceptible 0.3% tollerance. which is negligable.

I look forwards to hearing your oppinions on this subject and views on the study I have performed to come to the conclusion I have come too.

Timelord

[Kandy]

Mines been about 7% optimistic over the last 1200 miles.

[vRS Carl]

Has your's been optimised by Custom Tuning though Kandy (just kidding don't take it personally)

I found mine was optimistic by about 3-4mpg till i accurised it

Carl:thumbup:

[sms]

mine can sometimes over as well as underpredict but is usually within a few %. Leaving the car ignition turned on whilst the engine isnt running affects it too from what ive noticed.

[ric04vrs]

Timelord , you've told us how you've done it , but what were your avarage fuel consumption figures then ?

[Timelord1.9Tdi]

Timelord , you've told us how you've done it , but what were your avarage fuel consumption figures then ?

As it currently stands TRIP = 52.3mpg and BRIM2BRIM = 52.2mpg.

The trip sometimes shows less mpg then the brim2brim method then other time the other way around which furthers my new belief trip comps are acurate

Timelord

[Dr Zoidberg]

I'm quite surprised it was that accurate as both my Octavias overread by about 6%.

[Timelord1.9Tdi]

I'm quite surprised it was that accurate as both my Octavias overread by about 6%.

This is what mine was giving for the first 1000 miles I had it, until I standarised EVERYTHING that could be. As written in the first post. I did this religiously for the following 3,100miles which then gave me a stable +/- 0.5% varriation. This has now... not only changed my mind to believe Trip Comps are accurate, but now show's me the inaccuracy of the Brim 2 Brim Method, due to varriations in pumps and other varriations that are natural in fuel.

I entered this test to discredit the trip computer through correct... I use this loosely "Scientific" methodology and report the amount that it was over optimistic.

I also approach my studies with a open mind and recorded accurate data and come to a conclusion based on true facts optimising any varriables (Where Possible) and I have been supprised as I now believe that trip computers are accurate within a tollerance of +/- 0.5% and with the first 1,000miles (Obtaining a average of 6.2% over optimistic), have now shown me that Brim2Brim method is inaccurate unless you use the same pump, nozzle and filling station obviously which then the method will be accurate.

Timelord

The statement +/- 0.5% is the rolling data... my current average is 0.3% over Optimistic !!

[Dr Zoidberg]

On a single tankfull I agree it's quite easy to get an inaccurate reading doing Brim2Brim.

Even if you use the same pump and stop at the first click (or some other arbitrary point) there can be quite a bit of variation in how full the tank is.

There's a fair amount of frothing as fuel goes in so unless you go very slowly and allow the fuel to settle completely then it will always cut off "early". How early that is will depend on all sorts of factors including how full the tank was to start with.

That said , I usually tried to fill the tank until the fuel was visible in the neck of the tank and had settled which minimised errors and if you do a long term average with ten or more tankfulls then that error is greatly reduced.

On my Bravo the trip computer is giving a similar margin of error compared to the calculated figure over 4500 miles.

[sms]

also i dont think the trip computer includes the pipe to the tank, im sure i read somewhere that the computer is programmed to work out the actual litres in the tank and not the pipe down to it including expansion tank?

[Dr Zoidberg]

also i dont think the trip computer includes the pipe to the tank, im sure i read somewhere that the computer is programmed to work out the actual litres in the tank and not the pipe down to it including expansion tank?

That's true for the range remaining , but the MPG figure will be based on the amount of fuel that has been supplied to the engine which will include it.

[vRS G60]

On a single tankfull I agree it's quite easy to get an inaccurate reading doing Brim2Brim.

Even if you use the same pump and stop at the first click (or some other arbitrary point) there can be quite a bit of variation in how full the tank is.

There's a fair amount of frothing as fuel goes in so unless you go very slowly and allow the fuel to settle completely then it will always cut off "early". How early that is will depend on all sorts of factors including how full the tank was to start with.

That said , I usually tried to fill the tank until the fuel was visible in the neck of the tank and had settled which minimised errors and if you do a long term average with ten or more tankfulls then that error is greatly reduced.

.

Agreed - but won't this give a better calculated fuel consumption figure because you may not be actually brimming the tank? Thus matching the trip (which always seems to err on the + side).

I'm going to take some figures from a number of tank fills to see how my trip computer fairs. I suspect it's about 10 MPG out on the + side.

[daveyonthemove]

I can see how your methods would be reasonably accurate, except for the level you fill up to.

Stopping at the click isn't always going to be the exact same amount each time as you can't garauntee that the fuel entered the tank at the same speed, so it may click earlier or later than that last fill.

And the title Brim2Brim is incorrect if your not actually going to the Brim??

If you fill to the very top every time, then you know the tank has the absolute maximum fuel it can take, and must be the same every time you fill it.

Good results though

[vRS G60]

Just to go back to this, could the dealer alter the trip computer settings via VAG-COM to get a more accurate reading?

Or could someone near Burnley (cough cough) do it for me.

[MoggyTech]

Just to go back to this, could the dealer alter the trip computer settings via VAG-COM to get a more accurate reading?

Or could someone near Burnley (cough cough) do it for me.

IIRC it can be adjusted 8% up or down with VAGCOM

[Nathanio]

Over the last (just shy of 3000miles) in the Octy the avg trip computer reads shy not over. Average on trip is 45.5MPG yet actual is around 47.2 iirc.

My Toledo massively over read; according to Trip 2 it was doing around 57MPG yet actual more like 51.2!

[Kablam]

-wrong-

Edited 2 October, 2009 by Kablam
mis-infomation

[maffwilson]

Tyre pressures checked each time too?