The tire diameter:
Both transmissions are 1 to 1 in high gear.
I can't say about RPM of 4000 at 70 MPH.
I only turn my engine 4000 RPM getting onto freeways. Sometines use it as
a shift point.
You've also got a water cooled and there could be a big difference in the
power curve, I believe that my 1970cc Air Cooled reaches peak torque and
HP at about 3400 RPM. (If someonce corrects me I won't be offended). I
base this on the engine sound, it seems to flatten out between 3400 and
3900 rpm.
If you're getting decent fuel econemy and I'm sure you've got lots of oil
pressure then 4000 RPM sounds within reason.
But in reality my van will run an honest 80 MPH on nice flat raods with
little or no winds, the top of the green pretty much limits you to 70 MPH
or just slightly more. If you ever get to 80 MPH downhill or powering up
to approach a big hill you're probably getting out of the green.
Check your power curve and at seventy I'd say you should be at about 85%
of your power/torque range.
Many list members are more knowledgable about tire sizes than I am so I'm
sure you'll hear a lot more on the subject.
An one more thing to remember, your spedo drives off your front wheel.
You could have small tires on the rear and larger tires on the front.
It's best to match all four for rotation purposes.

Stan Wilder
83 AirCooled Westfalia