Wouldn't Doc Brown know how to make fuel?

I know how to make high octane fuel and it isn't very easy, especially in 1885. You'd need catalysts, high-pressure reaction vessels, and a distillation column. I doubt that Doc Brown would even have known the process well enough to know where to start.

..*.. TxMike ..*..

He didn't need to know how to make it. Gasoline was already commercially available at the time. It was first sold as a solvent in 1870, and the term "gasoline" dates to 1863. Prior to being marketed as a solvent it was still being made, but it was considered a useless byproduct of making kerosene.

Raising the octane of late 19th century gasoline enough for a modern engine to run properly might have been an issue; it depends on whether or not Doc Brown could buy or make some tetraethyl lead (TEL). Making it is difficult, but it was first made in the mid-1800s, so it wasn't impossible to get in 1885. But even if he couldn't get any TEL, and couldn't come up with any other form of octane boost such as ethanol, he could have retarded the ignition timing enough to make it run well enough to at least get to 88 MPH.

Or, he could have simply siphoned some gas out of the DeLorean that he arrived in.

Had it been a diesel engine, there wouldn't have been any problems at all. He could have used plain old kerosene. In fact, it was a pretty big oversight on his part to not use a car with a diesel engine, considering he'd been planning for a while to travel to the "old west" time period. A diesel engine can run on not only kerosene, but vegetable oil too.

Another option would have been to simply drive the car into the ravine, or off any suitably high cliff. In freefall, the car would exceed 88 MPH after falling for about 100 yards (the length of a football field). However, he would have had to make a pair of parachutes for Marty and himself so that they could jump out before the car hit the ground in 1985. Making parachutes shouldn't be too hard for someone like Doc Brown.