Not being familiar with the machine in question, the assumption I am making is that the speedo drives off the transmission just as in the later model machines. If that is the case, the OP's question is what happens to the READINGS, RPM vs speed. The ratios involved in the primary and transmission have not changed just because a different tire is installed, thus the RPM vs speed READINGS remain unchanged. It is the ACTUAL speed which decreases with the smaller tire. Perhaps the intent of the question is how much actual speed will change, but that is not how the question reads.

If the speedo drives off the front wheel, then the RPM vs speed will change because the front wheel is unchanged, meaning the speedo is unchanged and the engine RPM will be slightly higher for a given speed with the smaller rear tire.

 

You contradict yourself. Speedo will change if the front tire drives the cable or sensor. Either the tire's revolutions increase or they don't. If they do then the speedometer will read a higher value.

 

Good answer from CHASARMS (V-Twin Forum)


Tire A: The 140/90B16 tire is 140mm in width. 90 percent of that is 126 mm. So, that tire is 126 mm tall.

Tire B: The 140/85 tire is 140mm in width. 85 percent of that is 119 mm. So, that tire is 119 mm tall.

The RPM is affected by the circumference of the tire which is calculated as diameter * Pi (or 3.14159)

Tire A diameter would be 406.4 mm (16in *25.4) + 252 (2 * 126) = 658.4 (total diameter) So 658.4 * 3.14159 = 2068.4 mm in circumference

Tire B diameter would be 406.4 mm (16in *25.4) + 238 (2 * 119) = 644.4 (total diameter) So 644.4 * 3.14159 = 2024.4 mm in circumference

Percentages: 2068.4 / 2024.4 = 1.021. That's a little over two percent.

So to use your view 70 mph at 3,000 rpm would be 3065 rpm.