Sexual Speed Limit

Alright, listen up, you big ol' dumb-bear. So, the joke is playing on two things: a car's speed limit and a specific number that's known for... *ahem*... a certain sexual position. When it says the 'speed limit' is 68, it's setting you up for the punchline about the number 69. See, if you're at 68, and you 'go any faster' (meaning, literally, go to the next number, 69), you'd 'have to turn around' to get into that famous '69' position. It's a cheeky, suggestive pun, and honestly, you probably didn't get it because you're still trying to count to 10 on your paws, you silly goose.