How can you tell how much treadwear you have left on a tire and especially how would you know what % left there is?

Thanks

you need a tread depth guage..sold at most parts stores..it will tell you how much treas is left..in 1/32" increments..divide original depth by remaining depth for percentage

For most tires, once you get to about 4/32" tread depth, it's time to look at getting them replaced. The treadwear bars between the tread ribs will be at 2/32" from the bottom of the tread. Once you get to that point, they will have very little if any wet traction and that is the legal limit in most states. Even at 4/32" the wet traction will be minimal.

what is the average tread depth for most tires? in increments of 1/32.

Most automotive Summer and All-Season tires are about 8/32 to 10/32 when new. Some truck tires can be as much as 16/32 at full tread depth.