You are assuming the guy with the DSG car has the worst possible situation while the guy with the manual car has the best possible situation. People just don't make optimum shifts with a manual box, so the results you get with a manual will be lower than the published figures. The DSG will shift exactly the same in tests or in the real world so it will not have this penalty.
Beyond this both will suffer in various ways but I'm just saying when you keep quoting the 10% figure you are assuming the manual driver always makes a perfect shift. They do not, ergo the gap is less than 10%.
Well i've had a MK5 Golf 170 TDI, and i've also had a lend of my dads for a few weeks when he was on holiday and mine was in for some serious warranty work - same car but he had DSG and I had manual. 50mpg on mine and 45mpg on his. Same driver, same driving style (as much as the DSG will let you be the same), 10% difference in fuel economy. Maybe if the manual driver has the perfect shift then it could be more than 10%. The manual Golfs and Scirocco since MY2011 have had economy gear change prompts, and when i'm in an economically minded mood it pretty much agrees with what I am doing (hence no prompts).
I know my car well enough that under gentle acceleration, unless i'm getting a regen, it will suggest I change up to 3rd at 18mph, 4th at 24mph, 5th at 30mph and 6th at 40mph, and correspondingly it asks me to shift down if I go at less than these speeds in said gear. If i'm not driving it gently, I will change up at a speed that will ensure that I will hit the next gear at not less than 1900rpm, to keep the turbo happy.
Maybe we forget these things once we get used to DSG and the car is doing all of our gear change thinking for us, but a manual driven well will surpass a 6 speed DSG in economy by quite a margin unless you are permanently sat in 6th and cruising.
This is because there are more than 2 situations for considering gear changes, and DSG only seems to consider 2 scenarios (extremely early change up under very light acceleration or have the gearbox knock down a cog or 2 and have the car run to the red line under anything under light acceleration). There are times when you put your foot down moderately in the same gear because for the situation you want might only require a little spurt of acceleration and it's not worth knocking down a gear.
Lower gear ratios on the DSG don't help the situation either. It's 6th gear is like "5.8" when you compare it to a manual - that will always cost you more fuel.
Not an issue with the 7 speed DSG or 8 speed BMW auto box - the auto is more economical than the manual in that situation, more gears to choose from and higher top gear ratio.
There are plenty of situations when the DSG doesn't make the perfect shift because it doesn't know what you want to do next (such as anticipating a maneuvre that may see you stay in a gear a few seconds toavoid the DSG shifting up and then back down again), it best guesses with the amount of throttle you are giving at any given time.
DSG will give you an easy drag race from the lights, but on the economy side, for any given driver and their style it is on a downer. Experience and VW figures reflect that.
