Formulas don't seem to "format" in the traditional way here .. so I have posted a picture of formulae out of my file.
This is also good info to have for camshaft selection...
If you are running 205/60/R13 the tyre diameter is 22.7"
So at 60MPH (96 kph) with 3.55 it calculates to 3152 rpm
with a 2.78 then rpm is 2468 (Torana 6cyls did have this ratio with trimatics)
with a 3.08 then rpm is 2735
Have fun with the calculator