This label, which is for a car that car runs on both electricity and gasoline, presents the biggest challenge. This is one case where there may simply be no perfect solution.
While fuel economy always varies from one driver to the next, with a car like this it will vary enormously. If a driver only goes on short trips between recharging, the mileage will be much higher than if he drives hundreds of miles between charges.
To help a driver make the calculation himself, this label provides separate mileage for electric-only and gasoline-powered driving. What it lacks is some information on what the mileage would be for, say, 100 miles without recharging. That would give a potential buyer at least some idea of how the combined modes average out.
For a head-scratching problem, this is a pretty good solution, though.