Tire size question

[schwank]

Actually the 225 and 245 are the width of the tire.

Tire sizes work like this

225/40 - 18

tire width (millimeters)/tire sidewall height (% of width) - wheel diameter (inches)

So a 225/40 is 20mm narrower than a 245/40.

Each tire has a sidewall 40% of the width.

So 225/40 = 90mm sidewall height
and 245/40 = 98mm sidewall height

HERE is a link with more explanation, or just google 'tire size explanation'