I am confused why, when reporting mpg figures folk will assume that the old way of “brim to brim” is accurate and any discrepancy is in the computer.

Though I must admit I’ve never had a situation in any car where a trip computer read lower than brim-to-brim, how do you know that it isn’t the trip computer that is correct?

Look at the inaccuracies in the brim to brim method:

1) easy to get a variation in the “brim” level of at least 1 litre – on a 50litre fill that’s 2%. If you use different pumps the variation will be even more. You need to do it over 4 or 5 fills to even out variation

2) the tripmeter calculates distance travelled by counting wheel rotations, then multiplies by the wheel circumference – gives the distance travelled. But as tyres wear the circumference of the wheel reduces by around 3%. So your figure will depend on how worn your tyres are.

3) the forecourt fuel pump is checked by Weights and Measures so I think we can forget any variation there

4) fuel temperature – it expands when it gets warm so fill up on a cold day and you get more weight of fuel for your money!

On the other hand the computer uses the volume of fuel delivered to the injectors and the distance travelled. Or does it use fuel delivery rate and speed? Inaccuracies here are:

1) calculating the volume of fuel by adding up lots and lots of tiny squirts!! Any tiny error is soon multiplied

2) tripmeter as above

3) fuel temperature is more consistent but will be different to when you bought it.

I did some interesting accuracy checks on my 1.9TDi Octy!

On a long empty stretch of road (yes they do still exist!) I set the cruise control at exactly 60mph then zeroed the trip computer. After 3 miles the speedo was still on exactly 60mph, average speed from the computer was 58mph and my TomTom sat nav had varied 55 to 56mph. So how fast was I going? Does that variation transfer to the tripmeter? :eek:

I did a trip from Leeds to East Grinstead. Followed the sat nav for the whole way. Sat nav told me the distance would be 261 miles and when I arrived the trip said 264 miles – difference of just over 1%. In a similar experiment in my wife’s Micra the sat nav said 91miles and the trip said 98miles – difference of over 7%. Incidentally the front tyres on the Octy were only 2k miles old, the fronts on the Micra were down to 3mm tread.

For the record, on the trip to East Grinstead the Octy’s trip computer gave fuel consumption of 60.1mpg at an average speed of 60mph – I just had to check that! Filled up and calculated brim-to-brim mpg as 58.3.

Again for the record – over 3.5k miles my computer reckons I have averaged 54.6mpg. Using fill-up data works out at 51.2mpg.

Which is right? :confused: Dunno! But they are both good enough for me – my bank balance is much healthier than with my last car which has to be the ultimate test.

Speedo's are built with an overestimate built into them (on ford's it's 10%, it's less on Skoda's but it's still there) however the car does know EXACTLY how fast it's going, you can pull this up on the climatatronic by pressing eco and the top right direct filter button at the same time, the temp readings will change, if you rotate the left temp controll until the big numbers in the left temp reading show 19 and then the fan control so that it's showing 19 with a little 1 next to it the right temp readout will be reading the speed of the car (which should match the tom-tom).

Cruise control will try and keep the car at the desired speed but it will fluctuate slightly for hills etc.

I used to compare the onboard to brim to brim on my furby and I have to say they worked out pretty close to each other.

The reading on the climatronic reads quite a bit lower than the speedo , and is below a true GPS verified speed

Oh? it seemed in line with my tom tom but then I didn't really spend that much time starting at it, driving at the time and all...

I'd not considered discrepancies with the trip odometer until I started recording total mileage and trip mileage per tank and discovering they didn't add up! I tend to use the brim-to-brim method because I appear to be able to manipulate the trip, eg if I start off delicately on the throttle for the first 150 miles and then hammer it everywhere for the rest, I can still achieve a high average for the tank, whereas if I do it the other way round, I get a below average for the tank.

Chris

Could the difference in total trip distance between sat nav and car be due to changes in height (i.e. hills). Sat nav looks down on a map and can trace the distance in 2d but may not take into account the distance travelled vertically. So what looks like 1 mile on a map could be more if there are lots of hills etc.