260E axle ratio = 3.27:1 (300E is 3.07:1)
195/65R-15 tires = 837 revs per mile

Speed at 60 (1:1 top gear assuming no torque converter slippage):

3.27 x 837 = 2737

Any other revs at speed can be determined by linear proportioning, e.g. at 85 MPH:

2737(85/60) = 3877

Speed per thousand RPM = 60/2.737 = 21.92

The speed/rev relationship is established by speedometer gearing and speedo head calibration and is independent of actual installed tire revs per mile or installed axle ratio. An axle ratio change or installation of tires with different revs per mile will affect speedometer accuracy, but not the speed/rev relationship.

Duke