dpw: My aim is to rev lower at higher speeds.

You need to change your differential not play with different size tyres. I can never remember which way round it is, in terms of a 'lower' vs 'higher' diff ratio, but I currently have a 4.11 in my BMW. If I want better acceleration I need to go to a 4.45 which would be great around town but murder on the motorway with the revs being too high. A 3.64 will give slower acceleration but much lower revs at high speed.

As has been pointed out you are supposed to keep with 5% of the original size and as you move to a larger diameter wheel you use a lower profile tyre in order to keep the overall diameter of the wheel and tyre the same in order to not mess with the instrument calibration.