According to developer Psyonix, the only difference between the cars in Rocket League are their hit boxes—that is, the invisible 'box' around them that determines how they collide with things. Here's a tweet from July confirming this.
But is this the case? Google 'Rocket League car differences' and you'll find a mass of conflicting information. There's this elaborate spreadsheet by Tamoketh, which indicates that certain factors vary between cars including turning radius and maneuvering. There's no way to confirm the 'science' of the tests they conducted to compile these stats, but it's compelling nonetheless.
This video by 1edzeppelin seems to show differences in the power of each car's 'kick', and after watching it, it does seem like there's something going on.
And this video by Evil Tuinhek seems to show varying turning circles.
If the cars in Rocket League do indeed have hidden stats, this seems like a strange design decision. I've definitely noticed people online using the same handful of car types—especially the van-like Merc. But if there is a difference, it probably isn't major enough to mess up the balance of the game. You never feel like one player has the edge because of the car they're using.
We spoke with Psyonix President Dave Hagewood in August, and he told us that "statistically so far we have not seen one car beating another." Hagewood continued, saying the differences are more subtle than people think. "Some advantages people think they might be getting with a particular car, might be an advantage with their style of play, but it’s not an advantage with everybody else’s style of play."
What do you think?