Fuel Economy

[nunncm]

Hi

I often see people on this site asking about fuel economy of which the responses can sometimes vary so I thought I would jot my thoughts on the matter as to how I work out the economy of a my current car and then apply to a potential car.

In a nut shell, work out what your current mpg on a full tank of fuel by measuring the actual fuel used from the pumps to how many miles you get. Compare this to the published figures and then reduce (unlikely increase) by the percentage difference.

I have a 2005 2.0 TSF1 A4 Avant. It is a fantastic car which I love but it is thirsty. The trip computer states that it is averaging 31mpg where in fact I rarely get more than 400 miles to a tank. So doing the figures, based on what I actually put in the tank rather than using the trip computer, I get 26mpg on average.

To work out my real mpg I simply convert from litres to gallons (70/4.55) = 15.385 then divide the actual miles by this value (400/15.385) = 26mpg.

To me there are two questions here?

1. Why does my trip computer state 31 when the reality is 26 – this is 17% out.
2. Why is the stated MPG figure showing 36 when the reality is 26 – this is 28% out.

I’m putting question 1 down to the fact that the trip computer needs adjusting to calculate this properly. I’d be interested to know whether there are any others out there who have looked at this to see if you are seeing the same ratio mismatch. I have never known a car to work this out properly.

It’s the second question though which I find the most interesting.

On Parkers the formal MPG for my audi is 36mpg which leaves me with the question as to how this is calculated which I was enlightened to on an episode of Fithgear where they tested a couple of cars mpg against their published figures. What they did was to get in the car and drive them nonstop through a town, down a motorway, through the country etc etc until they ran out of fuel and guess what, the figures matched perfectly. So what grates me about this is how far from reality this actually is. People don’t drive their cars nonstop. They do several journeys to a tank of petrol (I do about 30) and each time one takes a journey their car will choke until it gets up to temperature. This is not taken into account when they come up with the mpg figures. So for my 20 minute 11 mile journey to work 5 days per week I have 4 junctions, 1 set of traffic lights and 6 roundabouts. The trip computer MPG starts from about 20mpg and gets to about 32 by the time a get to work. So using just these figures 26 would be about right.

So I think it is fare to say that my driving habit is closer to town than rural so expect it to be lower than average and as to my driving style I would put a little above average as I don’t thrash the car but I’m not a slow coach either. Anyway, now I am armed with the figure (28% less that published) I can apply this to any potential car should I want to change. For instance, if I sell by Audi and buy an equivalent 2.0 TDI diesel which has a stated mpg of 47, I apply the 28% reduction and get the realistic figure of 33.9 which should give me an extra 122 miles per tank leaving me £45 a month better off, although I would need to find another couple of grand to do this and would lose performance etc etc.

So if you are looking to buy a nice powerful car but have concerns about the fuel economy look at your driving habit. The less frequency of journeys and the more fluent (i.e less stop start), And the more long distant they are the closer you will get to the published figures assuming that you’re not heavy footed.

Also the last thing is obviously the amount of mileage you do a year, poor economy is more bearable the fewer miles you do.

Hope this helps.

Chazz.

[Madangler]

If it's any comfort, my B7 2.0 TDI returns on average 48mpg - measured!

Though on the trip computer, bearing in mind the same journey (70 miles) every day indicates 60mpg. I top up with 60 litres every 600 ish miles. it's great. And with that sort of return, you can afford to be heavy footed every now and then..... within speed limits of course.

[Monsieur]

Thats nothing - I achieved 200mpg going down a long hill the other day

[winnie070383]

Take your foot of the pedal and you get ----mpg

[kodkod.84]

Click the petrol pump on my signature and this will take you to a website called spritmonitor.de, here you track your ACTUAL mpg from inputting fuel up data and you can see the statistics of different cars and cars the same as yours. The DIS MPG is very optimistic unless you calibrate with VAG-COM.

[darich]

Click the petrol pump on my signature and this will take you to a website called spritmonitor.de, here you track your ACTUAL mpg from inputting fuel up data and you can see the statistics of different cars and cars the same as yours. The DIS MPG is very optimistic unless you calibrate with VAG-COM.

What's involved in recalibrating the DIS?
does it then give a more accurate reading?

Why isn't it set accurately when it leaves the factory??

[ss30]

My 2005 A4 2.0 TDI's computer say's I do between 5-7 MPG more than I get from working it out myself.

I think that it's dreadful that they over estimate buy this much, let's face it the computer knows how fast you travel, how far you travel and how much fuel goes in to the engine how the hell can it not tell you the correct MPG.

I have attached a pdf of my spreadsheet data.

ss30

[MeAudi]

Hi Nunncm,

What you say all makes sense and I applied this logic when I went from a B6 to a B7 A4 18 months ago. In my case, I should have got 2mpg more than before but in practice, I get around 3mpg less!

I went from a remapped 1.8T to a 2.0TFSI which has the same power and torque curves as my old car and the 2.0TFSI is supposed to be a more efficient engine.

However, a couple of things have mucked up the theory of what I should get.

Firstly, I had to go auto and the multitronic box seems to sap power when it's cold. So, shorter journeys in the winter use more fuel than before. I've already noticed a difference in mpg now that the sun is out.

Second, the B7 seems to be even heavier than the B6. I've had this on a weighbridge a number of times, with a trailer, and after deducting the trailer weight then the car is right porker and well over the weight stated in the literature!

So, next time I'm in the market, I'll be using the mpg website as suggested by kodkod.84. I think this gives the most accurate, real-life figures.

[drexel75]

Filled up today and worked out my mpg to be 26.6 which is better than the 24.5 that the DIS was showing.

[GarJE]

Bloody hell. I get 38mpg out of my 2.0t fsi saloon. Exactly the same as my old 2.0t pd (140). Granted, I think there was something seriously wrong with the diesel.