Rule of thumb is, every inch up, you take 5 off the profile....so 45 profile on a 17" would be 40 on an 18", and 35 on a 19" to maintain the best rolling diameter.

225 on a 7.5" is correct, you can also run a 225 on an 8", or a 235 on an 8" rim with no issues...just check for clearance in the guards.