Specs & Stats
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Tires and gearing
In Canada, most 40s and early 60s originally came with H78-15(B) tires, with an overall diameter of approximately 28.4 inches. They are usually replaced with 225/75R15 (approx. 28.3 inch) or 235/75R15 (approx. 28.9).
Taller tires reduce torque as it takes more torque to turn a larger tire. But larger tires can give better performance off-road (in mud for example). They also reduce rpm on the highway, giving better fuel economy and lowering engine and drivetrain noise. Of course they look good too! But if the tires are too large, highway performance on hills may suffer, unless you modify the gearing. Also, a wider tire increases the stress on the axle.
Physically, the talles tire you can fit on a 40, 60 or 70 without (much) problem, is 33". A width of 9.5" will not limit steering radius, but 12.5s will require modifications of one sort or another. But the size of tire you can run really depends on your differential gearing and how fast you want to go.
Canadian 60s came with 3.7 diff gears, and the 40s came with 4.11. Given that BJ60s and BJ42s of the same year have the same engine and transmission, the only difference is the differential gearing. That 10% or so difference allows 42s to run 33" tires comfortably (decent low-end torque and highway driving), while the 60s are limited to 31". Anything over those sizes will decrease performance too much for most people. Unless you don't want to go fast, ever.
To run taller tires, you need to lower the gears in the differentials to return some or all of the lost torque.
If you're considering wider tires, remember that the wider the tire, the harder the steering will be. This is important for those of us without the luxury of power steering.