We're all pretty lucky, really. I'm going to go out on a limb and say that everyone reading this was born in an era when access to automobiles was relatively cheap and easy. But let's take a moment and spare a thought for all those poor bastards unfortunate enough to be born before cars. I bet some of them could have been amazing drivers, right?
Now, I tend to put the origin of cars back a bit further than many, but just for the sake of keeping things easy, let's go ahead and say the era of (relatively) easily-accessible cars starts around, oh, 1900. So that would make our pool of potential but unavailable drivers anyone born from, say, 1830 and earlier.
That's a pretty good-sized pool. So who may have been an undiscovered driving savant? Isaac Newton I think would have had a good grasp of the underlying physics of driving, but I've never gotten the sense that he'd be able to translate that into physically driving. He seems sort of like a pants-wetter at any speed over 12 MPH. And I'm not sure how well horse riders would necessarily fare — driving a wheeled vehicle like a car is a pretty different beast.
Now, chariot drivers, they may be our best bet, especially chariot racers. They probably already understood about how to corner, and their solid-axled chariots likely made them skilled drifters, too. And the most famous of these was Gaius Appuleius Diocles.
Diocles won 1462 out of the 4257 races he competed in — while the ratio isn't incredible, it's a staggering number of wins and and even more staggering career. Being an ancient Roman charioteer wasn't exactly the most long-term of professions, but Diocles managed to do it from 18 to 42.
If you tally up his career winnings, it comes to 35,863,120 sesterces, which some classicist managed to translate into 2014 dollars and found it would be about $15 billion, making him the highest paid athlete ever.
So, yeah, I bet if you could get this guy behind the wheel of a car and train him for a few months, he probably would have been pretty formidable on the track. All that winning with just 4 real, sweaty, smelly HP? Think what that guy could do in a Porsche 917.