Depends how much you are static on your commute. For the EU testing cycle, the car spends 24% of its time at standstill.
I’m assuming that a GTD uses a litre of diesel an hour on idle - this seemed to be approximately right when my Scirocco 170TDI took 4 hours driving to do what is normally a 20 minute trip during the great Newcastle floods of July 2012 (I averaged 11mpg for the 12 mile journey, and I was switching the engine off when it was clear I wouldn’t be moving for more than 30 seconds).
The new “miracle” official mpg figures are based on the inclusion of stop-start tech. Take my old Scirocco 170TDI manual (presumably very close to a MK6 GTD, for which I do not have the figures), officially 53.3mpg on the combined cycle with no stop-start tech, and compare it to a MK7 GTD at 67.3mpg. for 24% of the testing cycle, the Scirocco is idling and the GTD has its engine off. The Scirocco uses 21% more fuel for the fact of having been idling for 24% of the test. Not all of those official gains are down to stop-start tech, but I would say a good 2/3 of it is. Brake regeneration plays a small part, weight savings are negligible as me opting for 5 door made the car heavier than a 3 door MK6/Scirocco.
In the real world, I estimate that I am stopped for a total of 90 seconds (6%) of my 25 minute journey. In that 90 seconds I probably make use of stop-start twice. If I’m going to be stopped at lights etc for more than 20 seconds I’ll let the car switch off, if not I will keep the clutch pedal dipped to keep the engine on. So the inclusion of stop-start affects about 3% of my journey (as for half my stoppages I do not let it switch the engine off). I read somewhere that unless your stoppage is greater than 20s then there are no stop-start savings in switching off.
There is nowt between my GTD and Scirocco in the winter, on my commute, and for saving fuel by switching the engine off on 3% of my journey when idling uses maybe 20% of the fuel in any given time that averaging 45mpg at an average speed of 45mph does. The impact of stop/start inclusion for me is saving 20% of 3%, or 0.6% on my fuel bill. This is the real reason there is almost no fuel saving between the “20% more efficient” GTD and my Scirocco.
I do wonder how many starter motors will have to be replaced prematurely on these stop-start equipped cars compared to old tech. Have car companies considered this and made them to last longer as a result? Doubt it – plenty of starter motors will be knackered at 50k miles I reckon. Admittedly it is the first cold start that puts the most strain on that starter motor, but even estimating that a warm start takes half the effort of a cold start, my starter motor is working twice as hard per commute with 2 stop-starts than none.
