Mr. Fusion powers the time circuits and the Flux Capacitor, but the internal combustion engine runs on ordinary gasoline. It always has.
9 seconds sound clip from the Back to the Future Part III movie soundboard.
You can hear this line at 00:38:26.438 in the DVD version of the movie.
Quote context
[...]
- If I never meet the woman, there's no possibility of a romantic infatuation, right?
- All right. We'll get the DeLorean and get ourselves back to the future.
- Yeah, it's no big deal. We've got Mr. Fusion, right?
- There's not gonna be a gas station around here until sometime in the next century.
- Without gasoline, we can't get the DeLorean up to 88 miles per hour.
- It's no use, Marty! Even the fastest horse in the world won't run more than 35, 40 miles an hour!
- Bartender said that's the strongest stuff they got.
[...]