Often we get the question of how long Route 66 is, and the different numbers out there seem to confuse people even more.
What needs to be emphasized is that Route 66 underwent many changes over the years. Some major examples are are the extension of Route 66 from downtown LA to Santa Monica, the removal of the Santa Fe and Los Lunas Loops and generally bypassing of towns and slowly getting to a state where Route 66 was being replaced by the Interstates.
Please note these are approximate numbers and you'll find more numbers out there. Also note that today you cannot drive any of those alignments continuously anymore, you need to mix and match the different alignments.
Data courtesy of Stefan Joppich
National Historic Route 66 Federation