I owned one. They don't, not in real driving conditions with the little 2-stroke engine screaming its guts out in a hopeless attempt to keep up with traffic. Maybe driving at 25 mph on level ground with a good tail wind you'd get that kind of mileage. Mine only did about 30 mpg or so.
I own one as well, in sedan form.
Mine gets 50mpg to the floorboards which varies from about 50mph up a hill in 3rd to 75 in overdrive on the Speedo if it’s flat and the winds correct . It’s the only car I own that putting 40psi in the radial tires (instead of the placard rating with bias tires) increases my top speed 10mph
My guess is you had a Van version, 3 speed, bias treads, wintertime, 20psi in the snow tires with the timing off, cracked distributor perhaps?
You gotta tune them all the time and the plugs have to be replaced often also. Wrong gap and you burn holes in the pistons.
Greasing everything about every 5000 is almost a necessity, but no oil changes, very different experience than a modern car where you just drive and don’t do anything.
Around town if I hit the kill switch going to every stop I’m in the low 80’s, car does get legit 60’s if your willing to go 55 stay alive on a longer trip
Even with a fuel leak I’ve never gotten below 40, but I rarely use it in the wintertime