Generalizations exist for a reason. They're generally true. The fact that you, and a few others might be an exception to ANY rule doesn't eradicate that rule. That is why the phrase "exception to the rule" exists.
The truth is the VAST majority of girls (AND even a lot of guys nowadays) couldn't care less about cars. The few that do don't get too involved with them. They may have picked up a thing or two about certain things through boyfriends, or family members, etc. but that's about it. This leaves us with the very rare, small percentage of girls who not only show an interest in cars, but have delved deep enough into understanding the science behind how everything works, understanding the variances in that science for the different makes and models, etc. that their car knowledge is credible.
Given this well established fact, you should be able to understand where your customers are coming from. Due to the slim chances of running into a girl who TRULY knows cars, most of your male customers will assume that you got the job because you know the bare minimum to get by, and/or are hot, and/or the store wants to be politically correct/fair/not sexist, so they had to hire a girl or two for image purposes.
This same mentality exists with ANY generalization. For example, obviously I'm pretty into cars. More of what I refer to as a car junkie vs. a gearhead. I love learning about the different designs, technological advancements in options/features, etc. Any make, any model, any era. Yet I defy the stereotypical behavior and lifestyle of most car junkies and gearheads. I don't have walls covered in posters and calendars of naked women, I actually despise alcohol as opposed to pounding beers every night, my place is always obsessive/compulsively clean, I'm pretty intellectual (into studying philosophy, religion, cultures/history, etc), concordantly, I'm a bit of a music and movie snob, and while I don't care what I wear when I'm working on a car, or at the junkyard, I dress pretty nice when I go out because I like to look and smell good (and girls certainly don't mind when you do, either). So depending on when people meet me for the first time, they have a hard time fathoming "the other side" of me. If I meet them at a junkyard or while working on a car, they assume I must be a bit on the hick/redneck/white trash side. When they meet me outside of that environment, they have a hard time believing I work on cars, or that if I do, I must not know too much about them because I don't "dress/look like a mechanic".
Likewise, I get the same generalization based treatment on the basketball court. Because I'm one of the few white guys on the court who doesn't pretend to be black (I don't wear shorts that go down to my shins, don't act or talk "urban", etc), most of the black players, and even a lot of the white guys, just assume I don't have an athletic bone in my body. Even AFTER I make a few acrobatic/athletic moves in mid air for a shot, or cross over behind my back when they lunge at the ball thinking they got an easy steal because "he ain't got no handles!", they still think it's a fluke. It's only after they've played with/against me a few times that they realize that's the norm for me, and accept the fact that despite "not looking it", I just might have some actual skills.
It's natural to take offense when the prejudice is used against you, but you have to stop and think about how often the generalization is true vs. how often it's not. Put yourself in the other person's shoes without your own biased perception of being a female for a moment. What would YOU think if you came across a girl claiming to "know" about cars? Would you say "I bet you do!!!!", or would you start questioning them to see just how much they "know"? Chances are, if you don't bring the bias of you/her being a fellow car girl into it, you'll be grilling her to see if she speaks the truth.
Me, personally, whenever I see a girl working at a parts store, I go into it HOPING she's knowledgeable about them, especially if she's attractive, because....well.....it's an ATTRACTIVE GIRL......who knows about cars!!! Please let it be true!!