The 275 45 is smaller in diameter then the 285 45.

The 45 is the aspect ratio, the sidewall height is 45% of the 275 section width.

The 275-45 has a sidewall height of 123.75mm.
The 285-45 has a sidewall height of 128.25mm.

The overall height of the 275 is 9mm smaller then the 285.

The 275 has to spin around 5.5 more times per Km vs the 285 (436revs vs 430.5revs)

__________________
'03 X5 4.4 Sport, last of the M62s (8-03 build date)
I believe in deadication to craftmanship in a world of mediocrity!