Just brimmed the tank for te second time and worked out the fuel consumption. The on-board computer says an overall on the tank of 48mpg. Working it out , the figure should be 42.59mpg. My wife's previous diesel Fabia's on board computer was almost dead accurate but this one seems unacceptably inaccurate. I thought the computer on my Mercedes was bad at 5% over-read but the Yeti is an 11% over-read. Anyone else found this?

PS. The poor consumption is probably down to the fact that the car is running-in and there were some problems with a noise on the car that necessitated some garage testing.

Edited 31 October, 201015 yr by survey

Mine's about 7.5% out.

A few years ago when I queried BMW about the same thing, my brim to brim results showed a 3-4mpg difference less than the OBC, they said that the OBC was the correct one, as it was the one that measured the flow of fuel into the engine, which it must do pretty well, to get the right mixture etc etc.

So, why the difference, I have no idea, but I have never had a car with OBC which matched my 'brim to brim' with the OBC.

Conspiracy theory: The petrol pump says 50litres but only puts in say 49.5litres. When we are doing our calculations we are basing it on what we are charged by the fuel companies. What if we are being charged for more than we bought? what is their margin for error? 10% like we are allowed with our speedometers?

Food for thought.

Mike

It does happen. Trading standards do spot checks I believe.

Pump delivery quantities are checked regularly and very accurately, I believe to less than 1%, so that is out of the window.

BMW bull**** Rockhopper.

Mine is consistently around 8-10% out which is pretty disappointing. It is pointless having the display when it is that far out.

Trading standards are very hot on measuring pumps so I would agree with Llanigraham that that is not the answer.

I have never had a trip computer that was accurate. They are all at least 5% optimistic. I have always found that they become less accurate the more journeys you do. ie if you use a tankful in a single journey you get a figure closer to the brim-to-brim than you do if you use the tank over a month doing 5 mile commutes. It is probably something to do with the way they store and consolidate the data - maybe a compounding of rounding errors.

Also 1 tank is not really enough to compare. You can never be sure that you filled the tank to exactly the same "brim" as last time and you can get anything up to 2-3 litres difference. On a 50-60litre tank that is a 5% error. Even using the same pump and stopping at the first cut-off is not accurate as small differences in temperature, in the angle of the nozzle and the insertion depth will cause variation. Do a running total over several tanks to reduce the error.

Another factor in the inaccuracy of fuel consumption is inaccurate measurement of distance travelled due to tyre wear. As you lose 4mm off your tread the rolling circumference of the wheel reduces which increases the apparent miles you are travelling. This is part of the reason why fuel consumption on a new car seems to increase as the engine "beds in" over the first few thousand miles.

I once tested whether the difference in speed between speedo and sat-nav translated into a difference in distance travelled. I set off on a trip to Cambridge - sat nav said it was 164 miles. Zeroed the trip and followed the sat nav exactly - recorded 164.8 miles! I then learned the Octavia trick of getting the speed display in the dual zone climate display and found that the car knew exactly how fast it was going - the speedo dial is programmed to lie!

I found an empty stretch of A road. Got up to 60mph on the speedo, zeroed the trip computer, set the cruise control and drove about 2 miles. Speedo said 60mph, digital readout flicked between 56-57, trip computer average speed said 56, sat-nav said 56.

Sorry! drifted off topic - must get a life