A few observations:
1) the initial prompt is a bit confusing. If they can capture no more than 10% of the market, but all the 5 companies have equal market share, 20% vs. 10%?
2) If mileage goes up, then demand goes down. This is a significant risk for the client, so think about how this impacts your calculation, and what other effects fewer people fueling up would have. Less ancillary revenue from gas stations, for one)
3) If capacity is a problem, consider licencing the fuel to competitors. Presumably they'd be down if the product is that much better, and if they can charge up to 30% more for it (the point at which customers are indifferent).