I think it makes sense to differentiate between basing the car's price in BTC vs the dollar. To me, basing the car's price in the dollar is less confusing than using BTC. For example:
If the car is valued based in USD - say, 50K USD, then Billy Bob would pay whatever is equivalent to 50K in BTC at the time of the transaction. So right now, Billy Bob would pay the dealership about 0.98 BTC. If the value of BTC goes up to 100K, well crap, that sucks for Billy Bob, but it is what it is.
Or you can base the value of the car in BTC - so let's say the dealership says the car's price is 1 BTC. The question arises if it always remains at 1 BTC or does it fluctuate depending on the value of the BTC itself, which is a lot more volatile than the dollar. It obviously makes sense to adjust the car's price relative to the BTC price. So if the dealer values the car at 1 BTC, and BTC goes up by 100%, then the dealer would have to adjust the price to 0.5 BTC. Or the dealer could be a jerk and keep it at 1 BTC. But then the dealer would be changing the car's price every day and that could get tiresome and costly for lots of cars and lots of volatility.
Example 1 seems a less confusing business model/practice. But it does suck for Billy Bob assuming BTC goes up in value over time.
In conclusion, please don't buy a car with BTC. Maybe 1 apple, but that's about it. Just HODL!