The warmerwagen mentioned:

And, the 2.0 I-4 turns out higher mph for the same rpms as a WBX.
In either the manual or shift.

For example, in both the 85 Automatic Westy and 87 manual Westy currently
available to me, 70 mph is 4000 rpm=I was surprised to find that the manual
has the same rpm as the auto.
For the I-4 2.0 powered manual shift Westy-70 mph is 3600 rpm.
And at 4000 rpm-80 mph. 10 more mph for the same rpm.

And I surmised:

If the overall gearing is the same (tire size&final drive&topgear ratio)
the revs/mile or rpm/mph will be virtually identical.

So if there is a ~10% difference it is either in the gearing or in the
measuring devices.

 If you can borrow a Garmin GPS you can eliminate the distance measuring
device error, and test both machines using the GPS.

Then it can only be the revolution counter differences or gearing
differences.  Tachometers can and do differ a bit.  Usually not 10%.

cheers,

pensioner