But the amount of bitcoin has changed. 

To calculate yield, you need to look at how many bitcoin do you have at the unwind of the leverage compared to the amount of bitcoin you hold had you not levered (which in the case of a car loan is the lesser number). You must take as a given a car will be bought and you will either borrow dollars or sell bitcoin to get it. You can’t compare to the scenario of doing nothing. 

What may also seem confusing is this type of leverage multiplies yield by less than 1 (minimizing lost bitcoin) which seems strange to financial types who can only see leverage as magnifying yield.