Remember, the VW’s extraordinary figure is the EC certificated figure achieved in prescribed 'simulated' drive tests. It has not been realised in real life by any of the journalists trumpeting it. The EC combined economy figures are now far more misleading than they were 2 years ago. The certification tests measure CO2 for tax purposes. From that the fuel economy is calculated. It's completely pie in the sky total nonsense. Compared to a bicycle, your idea would involve power losses and excess weight to store the power generated, but combined with solar panels there might be some mileage in a pedal assisted system.