Over the weekend I checked my tachometer reading at 60 mph indicated and it
read 3500 rpm.  This an '85 4-speed, with the 4.86 ring and pinion and the
0.85 fourth gear.  I am running Kirkland 195-75-14 tires, which have a
theoretical circumference of 80.16".   However, from the Tire Rack website,
the same size tires are listed as 816 revs per mile.
 The speedometer is marked as being calibrated for 805 revs per mile.
I have recently checked the odometer using mileage markers over a twenty
mile distance and it reads 4% fast.   It read 10.4 miles for every 10 miles
traveled.  If you use the 816 revs per mile figure from Tire Rack, it
calculates to a 3.8% error.  Which means both my speedometer and odometer
are probably both reading 3.8% fast.  So 60 mph indicated is really 57.8
mph at 3500 engine rpm.  The Bentley lists the allowable speedo error as +3
mph.  A  true 57.8mph can only read as high as 60.8mph on the speedometer,
which means my speedometer is  within the specs.
The difference between tire revs per mile and the theoretical circumference
calculated from the diameter given is curious.  I didn't realize the tread
contact area and tire bulge affected the "rolling circumference" this much.
Do inflation pressures then affect the tire revs per mile?

                                                                        Jay