The table on this link may help
http://www.tyresizecalculator.com/charts/tire-width-for-a-wheel-rim-size-chart
I know it starts at 5" but you can extrapolate back the numbers, so the MAX on 4.5J's would be 175's. I used to have 165 tyres on 4.5J's which were fine, but have changed to 175 on 5J (inch) width Alloys, again fine don't rub and my speedometer is more accurate!
Aidan