A properly designed gas tank will prevent the formation of fumes, whether by having a “bladder” within (most modern cars do this) or perhaps some type of venting.
I’d consider it far more dangerous to be going 40mph under the prevailing speed limit on a highway you can’t go around, such as over a bridge. The tank is a minor consideration.
I also believe you could find a way to use bladders and fairings in your tank design such that it bulges out into a 50gal tank when starting off and gradually stretches and shrinks back into the existing profile and shape as the gas gets used up. A further mileage hit at first, but all bonus miles.
Does a rider survive either one exploding?