Formula example(Use it for any tyres )
Old Tyres
255-55-18
MM
255 x 55 /50 =280.50
25.4 x 18 =457.20
Total dia. =737.70

New tyres
275-40-20
MM
275 X 40 / 50 = 220
25.4 X 20 = 508
Total dia. = 728

Difference =9.7 mm (-1.31)

Speedo interference: -1.31% .Will read fast!At 70 mph ,true speed will 69.08 mph