Need 40s as the 45s look shit.. But can i get 40s for my 15s

My wheels

 

40mm???? Do you mean 40 profile? If so the 40 refers to the percentage of the cross section that the tyre wall is. In other words if the width is 205 mm then the side wall height will be 40% of the 205mm)

If you're on 15's you should be running 195/50/50's dropping lower on the profile will more likely than not open a can of worms and look rather odd. I know when I put my XR2i wheels on my Escort it looked very bizarre on the 195/45/15's