Posh sprung the biggest upset of the Championship season so far, Michael Bostwick and Dwight Gayle securing their first victory since October to ruin Cardiff's perfect 10-game home record.
"We have beaten the best team in the league and also won at Hull so that shows we can beat any team in this league," Ferguson said.
"It's a very good day. But we haven't been consistent enough this season.
"I have always believed in the players and maintained the confidence that we can get the job done - but we need to produce consistent performances.
"We played very well but we did so last week. The difference was we defended well too.
"We stuck to the game plan and deserved the win but we didn't finish them off. We were hanging on a bit at the end.
"But if we continue to perform like that we will be fine in this league."