It follows a similar claim by City boss Roberto Mancini that his squad were on a par with United, man-for-man - despite the evidence of the table.
Wenger said: "Man City are a very good side who can compete with Man United.
"They dropped some points that were unexpected. Mancini is right and we saw that again when they beat Man United. This game is based on collective expression so it's difficult to compare man for man.
"We are 21 points behind. That is linked to our bad start. Against the big clubs we didn't do well. In Germany, Borussia Dortmund are 20 points behind Bayern Munich and they just beat Real Madrid 4-1.
"The same people that say we are 20 points behind United, may say Dortmund are the best team in Europe.
"And we beat Bayern Munich on top of that."