Government mandated safety equipment includes safety belts, a two-chamber master cylinder, an airbag or two, some sort of bumper (though nothing like the bumpers they used to make in the 1980's), and a steering wheel that will not impale the driver like a bug.
I don't think these items weigh very much, even though it is always a hoot to blame the government. I doubt that these things weigh 6% of the total weight of a Miata. One obese passenger would decrease mileage more. What decreases fuel mileage are weight, friction and air resistance. The new cars are all very aerodynamic. An old Volvo or a VW Microbus is like pushing a brick through the air.
Engines are MUCH more powerful than they used to be. My old '81 Buick Regal had a 3.8 liter V-6 and it had about 90 bhp. The same engine block in newer GM cars gets the same 18-22 mpg or better and puts out over 300 bhp.
I would think that 100 mpg would be about all you could expect from any internal combustion engine, 4-passenger capacity vehicle. Maybe there are fuel cell, hybrid or magical mystical turbines that could do better. I really hope there are.
For some reason, the Wankel engine, which just whirls around and around and has many fewer moving parts, has always gotten worse mileage than the internal combustion engine with all its many parts travelling in a variety of directions,
I do not know what that reason is, but I am guessing friction has a hand in there.