Thanks Rotarykid, reading that was an eye opener, there is a table on page 8 that lays out the differences and explains them below;
"As can be seen, diesels appear to perform the best with respect to their label fuel economy, outperforming the label by 4.3%. Conventional gasoline vehicles come very close to meeting their label, falling short by only 1.4%. Conventional vehicles with relatively high combined fuel economy (here assumed to be 32 mpg or more, representing the top 10% of conventional vehicles in terms of fuel economy) performed only slightly worse, falling short by 1.7%. Hybrids fall short by a much larger margin, 8.2%. Thus, the greater shortfall seen with hybrids appears to be more related to hybrid technology than to simply high levels of fuel economy.
With respect to the mpg-based label values, diesels still perform the best of the four types of vehicles, now exceeding their label values by 18%.b Those conventional vehicles with relatively high fuel economy fall next, followed by the typical conventional vehicle and hybrids. Thus, the YourMPG estimates indicate that hybrid performance differs from that of conventional vehicles, including those with high fuel economy."
The other interesting part is the mention of them downgrading the EPA reading by a further 9.5% as mentioned below;
"We also are finalizing an additional downward adjustment to fuel economy estimates within the 5-cycle method. We put in place a downward adjustment to account for effects that cannot be replicated on the dynamometer. There are many factors that affect fuel economy that are not accounted for in any of our existing test cycles. These include road grade, wind, tire pressure, heavier loads, hills, snow/ice, effects of ethanol in gasoline, and others. We are finalizing a 9.5% downward adjustment to account for these effects. The detailed technical basis for this adjustment factor is contained in section III.A.5 of this Final Technical Support Document."
Note the compensation for ethanol in the fuel, how much ethanol is there in D2?
Further reading shows that they know that they biased the fuel economy of the Hybrids upwards by using hot starts;
"On average, hybrid fuel economy was 11% less than the composite EPA label values. The average onroad fuel economy of the Toyota Prius vehicles was closer to their composite label values than those for the two Honda models. On average, the onroad fuel economy of the 14 hybrids tested varied more than the conventional vehicles. This could be due to hybrids’ greater sensitivity to operating conditions which can either take full advantage of the hybrid technology or essentially nullify it. The fact that many vehicles started out testing with a hot start likely biased onroad fuel economy upwards to some degree. Thus, the actual shortfalls found would have been greater to some degree if testing had begun with a cold start. "
So if you factor in the admitted 18.3% penalty as well as the 9.5% downward correction for all the ethanol we burn while always driving uphill into the wind while overloaded on underinflated tires in a snow storm, then I guess VW might have been right with its 50+ Mpg estimate. Take the EPA of 41 Hwy and add 27.8% and you get 52.4 Mpg. The people who achieve the high Mpg here make sure their tires are overinflated, loads are light and conditions are good before making their mileage runs so they will likely achieve over 60 Mpg. The other thing of note is that the "trip length" for the EPA in their formulae is 7.5 miles (we barely warm up by then) at an average speed of 19.6 mph in the city, now what type of vehicle would that favour - anyone????