My slowest car was a FIAT 500, built maybe 1968 with 14(?) hp. I bought the old thing for a few hundred bucks. It had an air-cooled 2cyl engine that vibrated about 10cm (4 ins) up and down in idle. With strong wind from behind it reached about 105km/h on a flat road. It was very reliable, although it had starting problems in winter due to the heavily worn engine with its poor compression.
Without that car, I could not have been able to travel to work, although I could barely afford it. It consumed about 5 1/2 liters per 100km.