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At 40 psi you actually have less tire patch when solo. Tire patch is that area of the tire that is sitting on the road keeping you glued to it. Imagine a Basket ball with you sitting on it with 7psi The flat area on the ground is more than if the ball had 14 psi. On the other hand if you have too little air then the tire will begin to cup in the middle and you will have less area because now the tire is more like a rim with the out side edges only on the ground. The difference between solo and two up is the difference of the weight of two people vs one person. Check the tire pressure when cold because if you are riding around you will heat up the tires increasing the pressure. It is an optimization of too little pressure or too much depending on the tire temp. I hope this helps. So it effects all three of your choices.
I ride a Tour Glide (not so much weight difference at one might think) and I have the best luck with 36 front - 40 rear regardless of the load. And that's per the book. I get 13 - 14,000 on a rear and last front I pulled off had 36, 000 and plenty of tread left.(got tired of looking at it and never had 2 new skins on the bike at the same timesince I bought it)
Check them regularly, especially when the weather changes. Don't deflate if you've ridden more than 2 miles and they check high.
The biggest thing is longevity. Running a tire low for as little as 200 miles will cause it to start dipping out (especially D402) and when that happens, it'll keep on dipping regardless of the pressures you run.
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