Hi.
You need to adjust the shift cable -- it's adjustment determines when it upshifts (and downshifts, too). A possibly oversimplified but nevertheless workable explanation is that the cable is supposed to indicate to the transmission the position of the gas pedal, and (in a manner of speaking) because it isn't adjusted properly it "thinks" you are pressing harder on the gas pedal than you really are, and is performing accordingly.

The tires have nothing to do with this -- although, noticing that you said they were studded (and therefore, are snow tires), it's always better to have the narrowest possible tire for the snow! Save the wider tires for summer driving.