sportrak tyres

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Hi all. I have just sold my series 3 and acquired a a 96 sportrak with a gen 60k on the clock. It has 225/70/15 continental contrak's fitted and they are poor at best when off road. Now what size tyres can i go to on these alloy rims as i would like a more comfortable 70mph cruising speed (less rpm)and a more aggressive pattern for off road. will i need to go up a rim size to do this Unknw the car has standard suspension with no lift so I cant go to big with them.
cheers

ive got 31x10.5x15 on my

ive got 31x10.5x15 on my sportrak i did have to lift it a little though, i would up the tortion bars on the front and made some new shackles for the rear, 3"lift or there abouts, plus the extra size of the tyres i think its about 7" higher than standard