360k+ wrote on 07/02/12 at 18:39:19:1987 model...
Transmission 4-speed constant mesh
Gearshift pattern 1-down,3-up
Primary reduction 1.889 (68/36)
Final reduction 2.956 (68/23)
Gear ratios, Low 2.214 (31/14)
2nd 1.500 (27/18)
3rd 1.095 (23/21)
Top 0.875 (21/24)
Transmission 5-speed constant mesh
Gearshift pattern 1-down, 4-up
Primary reduction 1.810 (67/37)
Final reduction 2.956 (68/23)
Gear ratios, Low 2.333 (35/15)
2nd 1.579 (30/19)
3rd 1.142 (24/21)
4th 0.957 (22/23)
Top 0.885 (23/26)
Overall, stock 4-speed, top: 4.886
Overall, stock 5-speed, top: 4.735
Overall, 4-speed with 5-speed primary, top: 4.682
So this gives us the following rpm @ 60 mph in top:
Stock 4-speed, 140/80-15: 4136
Stock 4-speed, 140/90-15: 3955
Stock 4-speed, 150/90-16: 3701
Stock 4-speed, 140/80-18: 3673
Stock 4-speed, 140/90-18: 3530
4-speed, 1.810 primary, 140/80-15: 3965
4-speed, 1.810 primary, 140/90-15: 3789
4-speed, 1.810 primary, 150/90-16: 3546
4-speed, 1.810 primary, 140/80-18: 3521
4-speed, 1.810 primary, 140/90-18: 3382
Stock 5-speed, 140/80-15: 4008
Stock 5-speed, 140/90-15: 3831
Stock 5-speed, 150/90-16: 3585
Stock 5-speed, 140/80-18: 3560
Stock 5-speed, 140/90-18: 3419
Not sure how tall or wide a rear wheel that will fit, but it is a fairly simple way to obtain substantial less rpm.