4mph would have been well within my margin of rounding, if you mean it works out to 120.
I'd take your spreadsheet but lack excel.
What formula do you use?
If you don't have Excel, my spreadsheet ain't gonna help you. I originally developed it in Quattro Pro, but I long since migrated it over to Excel.
Your formula isn't based on actual tire rolling radius. When I did up the spreadsheet I spent a lot of time with tire company brochures and web sites, finding the actual revolutions per mile for various tire sizes. Then I factored that in and created a speed-to-RPM chart for just about every tire size from 205/75-15 up through (IIRC) 33".
Yeah, I knew I wasn't factoring in actual height or revs per mile. For my purposes it didn't matter. I was just wondering if there was a more fundamental flaw with my formula.