Liza, I have heard that car odometers are pretty accurate, with the odd exception. When I did my calibrating, we had 3 cars at the time (now we have 2), and they ALL read exactly the same mileage for a 10 mile trip, and this was also confirmed by an online map-it program I mapped it out with, so I felt all that was a pretty good thing to rely on. Since then I have ridden 60 and 70 mile bike routes and then mapped them out online and my bike computer confirms exactly what the online program says the trip was. Good enough for me at this point!
Makbike- I found the charts were way off. Others I know have said the same thing to me. It's best to use the charts to get in the ballpark, then adjust it in real life.
But of course people do it the way they want to.