Saturday, February 20, 2010

Your Mileage WILL Vary

The man in the car waved at me. So did the woman in the next car. They didn't use all their fingers.

I think it was because I was driving on the motorway at 60 mph. The speed limit was 70 as far as other drivers were concerned but they didn't realise I was conducting a Scientific Experiment.

You see a few months ago I bought a Bluemotion Golf. It's wonderful: comfortable, quiet, solid German engineering, good to drive and well-equipped. And its fuel consumption is phenomenal: 60.1 mpg urban, 83.1 extra-urban and 74.3 combined. That's about 900 miles on a tank of diesel.

The problem is that I've been getting just 600 miles from a tank.

So I designed an experiment. I decided that in the Golf I would

  • Turn off the air conditioning
  • Drive half a mile to the local service station
  • Check that the tyre pressures were optimal
  • Go to the motorway and drive along at 60 mph to the next junction.
  • Drive back along the motorway and go home.

This should be closer to an "extra urban" cycle than a combined one.

I must say it is a little nerve wracking driving along the motorway at 60. One is constantly checking the rear-view mirror to see whether some speed fiend is burning rubber in his attempt to avoid running up your exhaust pipe, but I managed. The majority of the motorway journey was actually done at 50 mph, because of roadworks, which should have made the journey even more economical.

And then, as a scientific control, I did the same with my wife's Mercedes C220CDI. Here's what we found.

Have a look at what's happening here.

In the Mercedes, my little drive (shown in the purple bar) achieved pretty close to the official Extra-urban (red bar) economy of the car. It was way better than the Combined figure, which is what I'd expect.

But in the Golf, the fuel consumption is far worse than I'd been led to expect from the official figures. Although 80% of the drive was at 50-60mph on the motorway, I achieved barely more than the Urban figure and way less than the Combined one.

What going on here? To find out, my wife phoned our dealer, Martins of Basingstoke, who promised to get a technician to call back. She phoned three times, but we've had no call back from technicians.

This is really a pity, because in every other respect we're very happy with this Golf. But we bought it for fuel efficiency and it's just not living up to its claims and it appears our dealer couldn't care less.

I feel like waving at VW. Not using all my fingers.


Slim said...

The only figure that surprises me from the chart is you not being able to achieve the Golf's combined figure. Manufacturer's figures are set in a test environment and achieving anything more than the combined figure would be a miracle. They also tweak their ECUs to get the lowest CO2 output for the tests. AutoCAR magazine ran an article on this recently, I will look for it.

Also, how many miles do the two cars have on the clock? diesels loosen up a lot over distance and the Golf won't be delivering it's best for at least the first 10,000 miles when the engine will probably only just be broken in.

By the way, many car fuel computers are 5-10% optimistic too, if you're measuring using them make sure they are accurate (by measuring the old fashioned way).

Sean Haffey said...

Golf has 3,000 miles: Merc has 9,500.

Slim said...

In my opinion the Golf needs a lot more miles before it makes the numbers. Do another 3,000 miles and I expect you will see an improvement.

In the meantime enjoy the free road fund licence ;-)

Sean Haffey said...

Well, if it really takes 10,000 miles before you get near the claimed mpg, I think they should say so. I can understand a few hundred, but not many thousands.

Anonymous said...

I'd like to comment on your lack of fuel efficiency -- it sounds quite frustrating.

However, I can't get past the air conditioning. Isn't it winter? Haven't I seen spectacular photographs of snow on your blog?

Sean Haffey said...


You are right - it's winter.

However what I have in the car is climate control, which can be used to warm up or cool down, I believe, but affects mpg. SO I turned it off to be sure, to be sure.