Are you coming from Claremont, by chance? (The "50 miles east" line is a standard description the Claremont Colleges use for their location, that's what made me suspect that.)
If so, you'll likely take the 10 to the 605 to the 105. Each of those transitions will be backed up, even at that hour.
Have you looked into Ontario Airport? If you don't want to change your flight, you might be able to return your rental car there and grab a shuttle from Ontario to LAX. (It's been a while since I've flown out of Ontario, but back in the day SuperShuttle definitely went from Ontario to LAX.) You'll still have to deal with traffic, but at least you won't be the one driving. Also, if you let them know when you want to be at LAX, they'll have a pretty good idea about when you should be leaving Ontario.