I owned a ‘57 plate VW Golf 1.9 TDI 105 and was really pleased with the mpg, getting 60 to 65 mpg on a long journey at 65 to 70mph. I recently replaced it with a 12-plate VW Golf TDI 105 with the new smaller, higher geared 1.6-litre TDI engine. I was told (presumably by the salesman in the sharp suit? Doc) that the mpg would be “at least as good” and the manufacturer’s figures support this.

However, early usage shows a dramatic drop in my mpg to averaging 50 to 55 at best on a long journey, which is 15 to 20 per cent less than my old car. I have taken the car back to the dealer and they have done “everything they could” with no noticeable change. So I was concluding that the manufacturer’s details about this new engine were simply a lie. Then, last Sunday, on a return trip from London to Manchester, I filled up at a Tesco (with Texaco fuel) and got a figure of 67.3mpg door-to-door for the 200-odd mile journey. That’s at least 25 per cent better fuel economy than all my previous journeys in this new car and matches, if not exceeds, my old car’s performance. Can different garage’s diesel fuel be so different? Can an engine be so sensitive to the fuel? I normally use Shell, and it worked fine on my old diesel. I live in Manchester – does this mean I now have to go 200 miles to London each time to fill up with Texaco?
Rodger

I’m wondering, first of all, what miles your old ’57 Golf had on the clock, and what miles you have on the new car. It might be that your long run has cleared away a few cobwebs left by the previous owner. Being somewhat cynical though (what me!), whilst manufacturers have certainly improved their official EC test figures, real life figures don’t improve to the same degree, as many owners will testify. I would not feel that you could put it down to the Texaco/Tesco fuel though (I thought Tesco was always unbranded fuel?) Have a careful think though about how that journey was compared with normal motoring, how often you get into fifth gear day-to-day, and how well warmed-up your car normally gets, and maybe therein lies the answer? I’m presuming that you’re quoting computer mpg figures, by the way. Had you thought that one or other computer might be well out? Or even the speedometer, and you’re actually cruising faster than in the old car? Maybe after that long run you will start seeing better figures, although you’ll expect better mpg now, as the weather cheers up and the car warms up quicker. But here’s a challenge: next time you do another really long run, set the cumulative/trip mpg display when you start off and watch how it changes during the trip. You may notice it taking 40 to 50 miles of climbing steadily, to reach a sort of peak at which it will probably then settle, if you’re cruising at a steady speed. I went down to ‘The Big Smoke’ recently on A-roads and motorway and was showing a rather disappointing 50mpg plus; but after about 30 miles on the M25 cruising at probably only 60mph average, (you often can’t go a lot faster on the M25, if it’s fairly busy) the mpg had climbed quite quickly to 58mpg. So, running at 60mph average, versus 70mph plus, was showing a very big difference in economy. I don’t know whether that throws any light on your experiences, but I hope this offers some ideas. What I’m suggesting is the possible benefit of a good long run, particularly if the previous owner had not given the engine much exercise, and the better economy conditions of open road motoring with a really hot engine. I’d be very interested to know if your mpg now stays better, or even continues to improve, as I’m suggesting. So do let me know, if you can find time.
Best regards,
Doc Diesel 

To which Rodger replied:

My second long journey amazingly ended up with exactly the same result of 67.3mpg, door to door, covering around 160 miles, mostly on the M4, M5 and M6. What a scary outcome for me to get exactly the same result. But to achieve this I had to keep the speed on the motorway to under 65mph, and there were long stretches of 50mph speed limits on the M5 and M6 that pushed up the average mpg a lot. This car does not seem to do speed well at all – to drive at 70mph rather than 65mph will drop the mpg by as much as 10 to 15mpg. If I drove at 55mph, I can only guess that I may get around 75mpg though. It’s as if the car’s aerodynamics are poor compare to the older model and its creating a lot of drag. Or else it’s simply down to the gear ratio changes – but I am still doing under 2,000rpm at 65mph. But at 67mpg, I cannot really push the complaint too hard now. Thanks for your interest.
Rodger