I have never thought that the GPS would be inaccurate but perhaps it is.

For comparisons, below are my tach readings.   Dont know how accurate the 19
year old tach is !!
In my 84 Westy I get 75 mph with 4000 rpm which is where I cruise most of the
time.
If the bulk of the traffic is cruising faster I will stay with them at about
80.   Its not easy to hold with the a/c on though.
At 3800 rpm in mine it shows about 72 mph.
At 90 mph (which I dont often do !!) my tach is showing 4700 rpm or
thereabouts.

Once again, this is with 205/70 x 14 tires. 40 psi rear, 35 psi front.

cheers,  Steve