Math Question

Okay, my Dart originally had a 2.92 axle ratio. I've since swapped to 3.23's, but haven't changed the speedo gear yet. I believe that this means my speedo and odometer are approximately 10% off.

My question is this: When calculating MPG's, would I add 10% to the result to correct, or subtract 10% from the result? My tiny brain can't figure it out...