I don't know from cars.
Yet in my new WIP, one of the characters is good with cars. She even carries an OBD II Diagnostic Code Scanner around in her purse. Heck. She even knows what the darn thing is for and what the heck the little codes it gives out mean... and further what one might need to do, based on those codes, to make the car work. It took me almost an hour of research per paragraph even though the car references are not the most important part of the scene.
But that's what it's about sometimes. Because you know what? When I read the scene, once all the car lingo had been parsed, trimmed, and edited to remove the Hey-Look-What-I-Learned-itis that always creeps into the initial draft of such scenes... it sounded right. It read like the character. She was there on the page, just like she ought to be, unhindered by my own ignorance.
Know what else?
I still might have to fix it again. I have buddies who know about cars and when they read it they may well need to point out things I still have a little bit wrong. Once it sells, maybe my agent, my editor, my copy editor, or another writer reading it for blurbing purposes might notice more little details that are wrong. I hope not. I HOPE my research was enough, that I have it down, but with each correction, my character will sound more genuine, so that when you, the reader, finally get to look at the final draft, it is as clean and perfect as I can make it... and it might impress you so much that you email me and tell me how right it was and THAT... THAT makes it all worthwhile.
What weird little people we writers really are. ;)