maximum difference occurs at 45 deg lat where it is approx 11.4 deg.
That seems a huge discrepancy; surely the earth isn't that far from spherical?
edit: yes, it's way off.
At sea level, the relation ship between geocentric latitude C and geodetic D is:
C = arctan ( (1-F)^2 * tan D)
where F is the "flattening", which for earth is 1/298 approx
That means that the following are some examples:
D=0 -> C=0
D=30 -> C=29.83
D=45 -> C=44.81
D=60 -> C=59.83
D=90 -> C=90
So the difference 45 deg geodetic is 0.19 degrees - which is 11.4 MINUTES approx, not degrees.