Now then, just how accurate is the trip computer on a mk1 Octy?

Just recently started having to do a longer commute (600miles a week) and I've been getting avg. readings between 47 and 49 mpg on the trip computer. Seems pretty damn good and that's sitting between 50 and 60mph.

I tried to figure out how many miles I get from half a tank and working out my mpg that way, but obviously you can't precisely say when you've used half a tank and the needle seems to stay on half for ages!

the way i do it is -

reset the trip when the fuel light comes on.

put an amount in the tank - 3/4 tank. and keep the receipt.

then when the fuel light comes on again, check what the trip is at.

that way your going from a known reference mark,

to the same reference mark with a known quantity of fuel (on the receipt)

the trip tend to over estimate by about 2-3 mpg... bit optimistic.

 

I know ill get slapped by those who do the Brim to brim method,

my problem with that is, you dont know exactly where in the tank you started from unless you run it out of fuel,

so you cant know exactly how much is in the tank....

Why do you think that the fuel light a more reliable indicator of content than physically brimming the tank?

I found the BC info very accurate,mpg,distance...

In my experience, its always been more reliable for giving consistent results. I know fuel can slosh around a bit, but its always worked better for me,

over my 13 yrs driving a lot of different cars.

I spose my main thing is where abouts in the tank / filler pipe is the tank capacity measured to?

is there more, or less than capacity available.

and... how much is left in the tank when its "Empty".

with the fuel light, IMO, theres a more consistency to the figures, as it comes on when the float hits the same point in the tank every time.

I found the BC info very accurate,mpg,distance...

 what is the BC info?

I'd have thought the capacity would be measured to the bottom of the filler pipe where it means the main tank? I've not ran mine down until the fuel lights been on yet, I've not got that much trust in them! Had too many old Fords with 'dodgy' fuel gauges I think.

Anyone know why the computer over reads then?

I know ill get slapped by those who do the Brim to brim method,

my problem with that is, you dont know exactly where in the tank you started from unless you run it out of fuel,

so you cant know exactly how much is in the tank....

 

Why do you think that the fuel light a more reliable indicator of content than physically brimming the tank? 

 

 

I don't think Mac thinks the fuel light is more reliable - I think he (like my wife) doesn't understand the brim to brim method, "because the fuel you put into the tank is not the fuel you used to physically drive the 'n' miles". Hence his reservation of the brim to brim method (" you dont know exactly where in the tank you started from unless you run it out of fuel"), even though you don't need to know exactly where in the tank you started from.

 

For the original thread starter's benefit, the 'brim to brim' method works as follows: 

1. Start with a full tank of fuel. I fill the tank up until the pump nozzle automatically cuts out for the first time (with the nozzle pressed in as far as it will go).
2. Before driving away, zero the trip meter. 
3. Drive around for however long you want, using up fuel in the tank. As an example, let's say you drove 240 miles before wanting to refuel (two commutes for you).
4. Refill the tank completely (i.e. back up to the point where the pump cuts out first time), and write down the volume of fuel you have just put in. The volume of 'fresh' fuel that you just put in now, is identical to the fuel volume you used to drive 240 miles. Let's say you needed to put 30 litres into your tank to refill it. I then repeat from step 2 - 4 to keep tabs on my fuel usage.
5. Calculate your MPG by dividing the distance you drove (in step 3) by the amount you needed to refill the car with (in step 4). In this case, our fuel consumption was 240 miles / 30 litres = 8 miles per litre, or 36 miles per gallon (multiply 'miles per litre' by 4.5 to get MPG).

I believe the computer over-reads because it is calibrated for a non-turbocharged 1.8L engine. I also think I read somewhere that it is just a simple tweak with VCDS to get it to read correctly. Certainly the computer on my parents' mk4 golf is bang on in line with my own calcs. 

 

edit: ok, new post saying "with the fuel light, IMO, theres a more consistency to the figures, as it comes on when the float hits the same point in the tank every time." - I stand corrected!

 

Ironically the method is less accurate than the brim to brim method as I describe, because even if you assume the empty 'bing' comes on at the same point every time, you will never be the same distance from a petrol station each time the thing goes 'bing'. This means if you put 30 litres in, you aren't going to have 30 litres above the 'bing' point in your fuel tank.... The problem? When you put 30 litres in, you assume that the mileage to the next 'bing' point used 30 litres of fuel, when in fact you will have used slightly less than 30 litres, because you filled up from below the 'bing' point. 

Edited 20 January, 201412 yr by Guest

ah ok, seem s i have mis -interpretted the brim to brim 

 

ie amount fuel in a distance,

 

rather than my way - distance to an amount of fuel used.

 

same calculation - just a different unknown in the equation

Edited 20 January, 201412 yr by mac11irl

I would think it is optimistic just to make the car look more efficient to people who aren't going to bother calculating it.

 

Just regarding the brim to brim... It is not without its flaws. As with any similar experimental procedure there are going to be errors introduced if a different pump is used e.g. higher pumping rate will cause it to cut out earlier by design of the mechanism and difference in the rate at which the petrol flows down the tube and settles in the tank. So, if you're trying to be very accurate, it will have to be carried out multiple times and averaged.

Then you've got temperature-volume variations.... Ha, I'm only messing at this stage. Realistically though they should give a reasonably similar answer (neither of which will be correct).

 

Ironically the method is less accurate than the brim to brim method as I describe, because even if you assume the empty 'bing' comes on at the same point every time, you will never be the same distance from a petrol station each time the thing goes 'bing'. This means if you put 30 litres in, you aren't going to have 30 litres above the 'bing' point in your fuel tank.... The problem? When you put 30 litres in, you assume that the mileage to the next 'bing' point used 30 litres of fuel, when in fact you will have used slightly less than 30 litres, because you filled up from below the 'bing' point. 

 

ah... you that why you reset the trip, when it bings. the fuel thats burned between the bing and fill up is replaced when you put the fuel in,

so if you burn 3l from bing, add 30litres that 27l until it bings again, post fill. but you'll also have the miles covered by the 3litres registered on the trip.

 

thus 30l worth of miles coverd from bing to bing