I agree with above, ive just come to accept that its slower going in the winter, i run two different bikes in the winter both MTB frames one with slicks and the other with 2.2 in the back and 2.1 with studs in the front and ive noticed its slower but only a little (around 6-7mph overall) I any more just completely ignore my speedo its too damn cold to care how fast im going
I run my knobbies at max pressure, until it snows then i bring them down according to how much snow falls that helps a lot.