As requested by NC Steve, I went out today and got some speed readings from my GPS receiver and noted the engine speed.
Due to weather conditions, I couldn't take it much above 75, but it appears that 5th gear is 500 RPM for every 5 MPH.
This is a '79 R65 with a 31/9 stock final drive, and Bridgestone Spitfire tires.
3rd gear
30 mph (48.27 kph) = 3000
35 mph (56.30 kph) = 3500
40 mph (64.36 kph) = 4000
4th gear
45 mph (72.40 kph) = 3750
50 mph (80.45 kph) = 4000
55 mph (88.50 kph) = 4750
5th gear
55 mph (88.50 kph) = 4000
60 mph (96.54 kph) = 4500
65 mph (104.59 kph) = 5000
70 mph (112.63 kph) = 5500
75 mph (120.68 kph) = 6000
