What happens when your odometer only has six slots and goes beyond 999,999?

it will go back to 000,000 and start again.

When if you exceed the limits of the odometer, in this case by driving one million miles, the odometer will start again at zero. Before the 6 digit odometers became common, you would see many cars that were over 100,000 miles and the odometer had "turned over". Million mile odometers were not common until the early 90's, so you will still see cars with the 5 digit odometer!