I think if surveyed at least 90% of native English speakers would understand "I want to wash my car" to mean a full size automobile. The next largest group would probably ask a clarifying question, rather than assume a toy car.
Yes, but you're speaking to a computer, not a person. It, of course, runs into the same limitations that every computer system runs into. In this case, it's undefined/inconsistent behavior when inputs are ambiguous.
Humans have the ability to reason and think critically, so it's pretty trivial to answer unless you think you're getting tricked by a riddle and the answer is the non-intuitive one.
You think that the reasonable interpretation of the question is that I want to go to the car wash but not to wash my car there, because I plan to wash my car at home?
> I Want to Wash My Dog. The Dog Wash Is 50 Meters Away. Should I Walk or Drive?
I dunno, that seems pretty clear to me still. Of course the answer to the question is now less obvious, since you can walk your dog to the dog wash but not walk your car to the car wash.
Sure, there are alternate explanations of both sentences, but there is one simplest and most straight-forward explanation. A system that assumes an explanation that is not the most clear, and does not ask clarifying questions, has room for improvement.
If things need to be exactly stated in a structured format that leaves no ambiguity, we already have programming and query languages for that.
Now why anyone would wash a toy car at a car wash is beyond comprehension, but the LLM is not there to judge the user's motives.