what should the rpms be for driving at 60mph ? on 1999 Mazda B3000

truck seems like its not shifiting into overdrive when driving above 50 mphs. push button overdive on automatic gear shifter doesnt seem to do anything could this be a problem with the push button or transmission overdrive light comes on ond off when i push button but no affect seems to occure

Asked by for the 1999 Mazda B3000