This is easy to do, they just make the fuel tank larger.
I could build one that could go a thousand miles, but wouldn’t have a back seat, and the springs would have to be stronger.
lol
VW made a splash with the Passat turbodiesel, able to do 700 miles on one tank of fuel. Perhaps jealous, Mercedes has recently introduced the E250 diesel, a sedan that can travel a bladder-busting 800 miles on a single tank.
Back in the early 70s, when the oil embargo hit, GM began working on a 40 gallon gas tank for the full size cars but the govt put the kibosh on it.