I was thinking some kind of glider or hover-cart/-bike. How does he expect to have any kind of speed on the ground through a medieval world with extremely lacking roads? Can prolly drive faster than horses, sure (dunno how fast horses can go when travelling for months). But I would be surprised if going over 50 with that car was an option due to the ground surfaces.
Should just have figured out how to make the cores expel liquid or gas to make thrusters, then build some simple glider that lets then fly directly to target ignoring roads entirely - their poor surfaces and twisting and turning.
You can land in pretty much any field without issue (and semi-safely even in rougher areas if it's an emergency - but expect the gliders to break and need repair by the craftsman conveniently brought along in luggage), and he even has storage magic to deal with any weight issues.
