Speedo gear chart vs. math...

A pal had a 27 in her Challenger from way back when it had stock tires & a 7 1/4" rear.

3.23 8 3/4 added, 15" tires too.

Chart said she needed a 33 with her combination so I swapped one in & it is now spot on at 60 MPH indicated & with the GPS too.

I suck at math.