255/40-18 should be a 26in tall tire.....not 28 ...although tire sizes arnt always what they say they are
a 3.90 gear and 26 in tire and pg ratio of 1:1 would give you around 3024rpm at 60mph
i use this to determine rpms...
http://www.csgnetwork.com/multirpmcalc.html
and this for tire sizes
https://tiresize.com/chart/
that could possibly be a ratio tag on the diff bolt