Tires can make a difference, but it's not just a matter of the width. I went from some 28mm tires with very stiff sidewalls (Armadillos) to 37mm tires that were more flexible and found there to be significantly less rolling resistance with the new tires even though they were a little heavier. An aggressive tread also leads to energy losses and slower speed - and frequently wider tires have a more pronounced tread pattern
An increase of 2.5 mph seems like a lot, but it's hard to say if the tires could make that big a difference without knowing which specific tire you had before and which model of Marathon you're using now.