Should be something like GVWR * max % per axle / 2, but I am afraid I won't even get close to the actual index using this formula. Or is it safe to assume that this is just the same as the tires load index (95 ~= 1521 lbs)? The reason for asking is that I am planning to stop using 20" wheels for everyday driving and looking to buy aftermarket 19" wheels for summers (I already have native polestar 19" for winters and thinking of some other design for summer). I really like my polestar 20" wheels, I really do. However I find them limiting my car's usefulness in everyday driving because of rather low profile tires. Each time I go on a long trip, or have to drive on unpaved roads (usually <5 miles to a hike's trailhead), or simply go over a rough road patch I mentally kiss a goodbye to these tires & wheels. It seems that it is just a matter of time to hit a stone or a pothole large enough to push through less than 2 inches of rubber and damage both the wheel and the tire. Taking a spare tire for long drives helps to relieve some of the worries but still, way too much attention is spent scanning the road for hazards and imperfections... Moving to 19" seems like a win-win: less worries in everyday driving and a dedicated 20" setup for the race track! Also maybe these 19" wheels would even allow me to go runflat (the OEM wheels do not have the extra groves to sit the runflat tires properly).