Using info on this page:
I found a 20 mile or so straight section of highway to test the odo on my four banger.
GE measured distance = 20.17 miles
ODO = 19.8 miles (195/60/14 Kumhos @ 44 PSI) = ( - 1.87 % )
Turns out that the odo is showing 1.87 % LESS miles than actually traveled....with tires that are supposed to be around 1% undersized.
So the ODO would be showing around .87 % LESS miles traveled with OEM sized tires on it.
* had it wrong....