"An ICE car with a range of 500 miles, could in theory get 1500 miles from the same weight of hydrogen, as the tank full of fossil fuels."

Cool. Now, given the same weight of hydrogen, and as typically stored at 70 MPa (10,000 psi) (per the Mirai), just how large would that tank (excuse me, tanks) have to be? Or perhaps more to the point, how many of them would I need?

By my estimation, it's a good thing you said "in theory", because in reality I'm not sure there's adequate room remaining for passengers in that vehicle.

Given a Mirai gets about 300 miles on its two H2 tanks, you'd need, what, eight more of them to store 1,500 miles worth of fuel?