OK, so the numbers in Sansone's book are correct. I was suspicious
because the gear ratio was the same for pre- and post-GOH
cars, while the motor horsepower was increased.
Given these numbers, obviously the cars with the lower gear
ratio, which appear to be all of the 75' cars, will have the
higher ultimate balancing speed.
During initial acceleration, up to about 18 MPH, both cars will
be about even since the propulsion system regulates the rate.
The R-40 and its higher-geared cousins should then start to
pull away because they have greater torque available, however,
they will top out at a lower speed than the R-68, assuming
level track. On any kind of upgrade, the R-40 should have
an advantage.