I made a tool that can help anyone who knows their speedo drive gear, tire specs (i.e. 275/40/17), and rear gear. It will calculate the exact driven gear needed and round that to recommend the nearest tooth count. Then you can input what you actually choose (i.e. 21 instead of 23), and it will tell you how far off your speedo will be at any speed.
I also use it all the time for gear ratios & tire sizes. I've shared that gear calculator here. I've gone back to this for every build, every trans swap, every rear gear swap, every change in tire size, etc... I've found it extremely useful. I've also collected data over the years on various transmission gear ratios, rear gear options, and available tire sizes of interest to me which are below the calculator in the spreadsheet.
It may look complicated at first, but it's very simple as long as your computer can use excel. Just stick to the highlighted cells. Only the highlighted yellow cells are meant to be changed. You'll get an error if you try to change anything else. I've protected it without a password. You can unprotect it if you want to make modifications to other cells.
<Link to the file on my share drive>
I think you will need to download it and open in excel. Google sheets may not be interactive.