What size is a 40 inch tire in metric?

See the table for sizes between 39.5" and 40.5".

What wheel sizes come with 40 inch tires?

40" tall tires are made for 20", 22" wheels. A bigger wheel means a shorter sidewall for the same overall height.

Tire sizes with an overall diameter between 39.5" and 40.5", grouped by wheel size. "40 inch tire" means the overall height, not the wheel.

Size	Diameter	Width	Sidewall
40x13.50R20	40.00"	13.50"	10.00"
40x15.50R20	40.00"	15.50"	10.00"

Size	Diameter	Width	Sidewall
40x15.50R22	40.00"	15.50"	9.00"

What size is a 40 inch tire in metric?: See the table for sizes between 39.5" and 40.5".
What wheel sizes come with 40 inch tires?: 40" tall tires are made for 20", 22" wheels. A bigger wheel means a shorter sidewall for the same overall height.