Chapter Seven: The Massacre at Hawkins Lab - S4-E7
Plot hole: The upside down should be mirrored, i.e. all left is right and right is left. When the team finds a gateway where Chrissy died, they should be seeing a mirror image through the ceiling. Also, all buildings should have been reversed. Directions to get there on the bike would have been confusing and reversed, and when any writing was seen in the upside down, it would have been backward.
Answer: They're both muscle cars, but that's about the extent of the similarities. But the 70's were filled with a lot of muscle cars, so some car models may look a bit like another's. Billy's car is a late 70's Chevy Camaro (probably a ‘79). Mad Max's car is a 1973 Ford Falcon XB GT. Incidentally, the Ford Falcon was produced by Ford Australia and wasn't an American car, or even sold in America (as in not sold at dealerships).
Bishop73
According to the experts its a Camaro '78 with added vented side fenders.