Plug-in hybrid electric vehicles (PHEVs) enable their drivers to choose whether to use electricity or gasoline, but this fuel flexibility benefit requires the purchase of additional battery capacity relative to most other vehicles. We value the fuel flexibility of PHEVs by representing the purchase of the battery as the purchase of a strip of call options on the price of transportation. We use a Kalman filter to obtain maximum likelihood estimates for three gasoline price models applied to a U.S. municipal market. We find that using a real options approach instead of a discounted cash flow analysis does not raise the retail price at which the battery pays for itself by more than $50/kWh (or by more than 15%). A discounted cash flow approach often provides a good approximation for PHEV value in our application, but real options approaches to valuing PHEVs' battery capacity or role in climate policy may be crucial for other analyses.