Here's a little helpful website.
http://www.bigcustomwheels.com/info_specs.jsp
Now punch in the size you are keeping, then play around with the other one til the rolling circumference is almost identical. For example, my rear tires are 255/40/17 on 9" rims, I'm keeping those and putting narrower wheels and tires up front to keep in the fenders, in my case, 225/45/17's on 8" rims. If you plug both of those in the calcutor on that website, you'll see they are almost identical. You can also play with offsets on there, but if we're still explaining what sidewall numbers mean here, we'lll save custom offsets, shortened control arms and mini-tubs for another day.
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