I read in the May 2010 Rider Magazine Tech Q&Q that our inaccurate mechanical speedometers are independent from our mechanical odometers, and so the odos should still be accurate. The guy, Chirs Sidah, who writes the column, says that the only variable is wheel diameter. So, if you are trying to figure your VN750 gas mileage, the odometer will accurately allow you to do that, but if you want to know your speed, you are on your own.
I guess that also means that if you swap tires to make the speedo accurate, you will screw up the odometer.
Digital speedometers are a different matter, and both speedo and odo are run by the same mechanism, so this does not apply.
Chris did not specifically mention the VN750.
'95 Kawasaki Concours
Sold my Vulcan
I do LOTS
of dumb stuff. Riding is only one of them.