So I need a little help with the numbers on this - I'm only a humble googlexpert on rocketry, and I just couldn't find a lot of the propellant facts. Still:
In 2016, Voyager 1 is about 11.7 billion miles or 18.8 billion kilometers from Earth.
It was launched by a Titan IIIE consisting of two UA1205 sold fuel boosters (about 192,000 kg of propellant), a first stage powered by a LR87-11 engine (guessing from the apparent size of its fuel tank 100,000kg of propellant and oxidizer) and 2nd stage LR91-11 engine (guessing another 100,000 kg), a 3rd stage Centaur-D (20,830 kg of propellant) and a little-known 4th stage Star-37E (about 1000 kg of propellant) for a total propellant load of about 606,700 kg.
Now you can fairly object to comparing rocket propellant to the fuel for a jet turbine or internal combustion engine, since rockets have to carry their own oxidizer with them. Also, those various propellants all had different weights per unit volumes.
But accepting those objections as not actually changing the ultimate conclusion:
18.8 billion kilometers travelled on 606,700 kg of propellant works out (according to my calculator) to 29702 kilometers per kg of propellant, or (assuming on liter weighs on kilogram) something like 11.7 billion (11,700,000,000) miles on 160,273 gallons of propellant, or around 73,000 miles per gallon.
This makes Voyager 1 the most fuel-efficient powered vehicle in history.