My car manual says I should increase my tire pressure in winter why

That's not true. Increasing your tire pressure reduces the amount of tire tread that's in contact with the road surface. Your tires should always be inflated to the proper pressure as shown on the sticker attached to the vehicle. Now if that sticker says COLD tire pressure specification you will want to increase that by 4 PSI if you've driven a mile or more before you add air. That's because the tires are then warm. Hope that helps! Jim

Higher air pressure can reduce hydroplaning but then again airing down can greatly increase snow traction. Who said this?

The NHTSA has a formula to estimate hydroplaning speeds. 10.35 x the square root of the tire pressure. Don't exceed the safe tire pressure rating on the tire sidewall. If hydroplaning is a concern get new tires that have a good wet traction rating and slow down!

Hope this helps

I've never heard of that before. Maybe contact the tire manufacturer. I'll be curious about what they tell you! Jim

You normally should add air in the winter because when it gets cold the tire pressure decreases due to the laws of physics. They need to be properly inflated when cold and not driven. Refer to the sticker on the door post for the proper air pressure.

Yes out as you see in the photo the PSI changes at different winter temperatures