I think what Dennis is saying is that the relationship between the tach and the speedo don't change with tire size.  That is strictly a function of gear ratios. If the engine spins the tire one revolution per 4 engine revolutions, that doesn't change with tire size. Only the actual distance traveled changes.

Keith Hughes
'86 Westy GenV (Marvin)
 ------------------------------
 
 Date:    Thu, 4 Jan 2018
 08:50:00 -0800
 From:    David McNeely <davmcneely40@GMAIL.COM>
 Subject: Re: Locking differential
 
 Dennis, I believe that an increase in
 tire diameter, which is also an
 increase in tire perimeter, does alter
 the speedometer and the tachometer,
 slightly or more depending on how large
 the increase is.  What the cable
 counts is rotations.  A larger
 tire means fewer rotations per mile,
 therefore both the tach and the
 speedometer register lower than with the
 factory set up.  In another
 vehicle, I have been stopped for speeding when
 I had oversized tires.  The
 officer and I had a discussion about that, and
 he let me off with a warning due to
 that fact.  And it is a fact, unless my
 mathematics education was wrong. 
 If this is wrong, please advise me how it
 is wrong.
 
 This does not alter your reasoning
 regarding the tire the original poster
 is running, or the other sizes you
 mention, because of the nominal increase
 over factory size.
 
 On Wed, Jan 3, 2018 at 7:51 PM, Dennis
 Haynes <d23haynes57@hotmail.com>
 wrote:
 
 > A 205/70-15 tire is ~26.3"
 diameter which is a very mild increase over th=
 e
 > original size of 25.6". This is
 about 795 revs/mile. The reading between
 > the tachometer and speedometer
 does not change with tire size, only how f=
 ar
 > or fast the vehicle actuals
 travels does. A lot of folks go  with 215/60-=
 16
 > or 215/55-17 as they are also this
 diameter. I like to 225/60-16 or
 > 225/55-17 to get bit more diameter
 and width along with increased weight
 > ratings. Syncros 235/55-17 is
 becoming a favorite. So, at 30 mph in town
 > speed what is the problem with 3rd
 gear and 2,400 rpm? Sounds like a swee=
 t
 > spot. 3rd gear is almost 1 to 1
 leaving the final drive as the real gear
 > reduction from engine to axles.
 4th is an overdrive, the output spins
 > faster than the engine which
 further reduces torque, leverage in wrong
 > direction. I think you probably
 have a good reasonable setup with the 2.2=
 .
 >
 > Dennis
 >
 >