How many miles per gallon can you get in a twelve passenger ford 350 van on the highway?

The fuel economy of a twelve-passenger Ford 350 van on the highway will vary significantly depending on several factors: the specific year and model of the van (as they've changed over time), driving conditions (speed, terrain, wind), how well it's maintained, and the load it's carrying (more passengers and cargo = lower MPG).

However, you can expect significantly less than 20 mpg on the highway. A reasonable estimate might be in the 12-17 mpg range, but it could easily be lower depending on the variables mentioned above. Checking the EPA fuel economy estimates for the *specific year and model* of the van is the best way to get a more accurate figure. You can usually find this information on the vehicle's sticker, the Ford website, or sites like fueleconomy.gov (for vehicles sold in the US).