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 > >