^^^^^^^ this.
And don't bother with cheap Jaycar box as they don't work. Need to spend $250 - $300 on proper speedo corrector kit. I haven't bothered as I know mine is 10% out and I'm used of it and have better things to spend 300 bucks on.
You could do it with tyre profile manipulation.
I would do the diff first, then work out what rim size you want. then using a tyre size calc adjust tyre profiles to suit the % you're out by.
It's not generally done this way, but you're looking at replacing your shoes.
FYI: Law states you must keep rolling diameter within +-25mm.