I have the 13/42 sprocket and matematically I should lose about 7-8 mph. But because we do not hit the rev limitor in fifth gear with stock sprocket, and with the new set of sprockets I do now hit the rev limitor. In speed comparison, I have only lost four mph according to my gps. I was doing 71-72, now its 67-68. Stock ratio is 2.86 and I am sitting at a 3.23 right now which is roughly 12% difference. It does rev out quciker thru the gears, but I am eatting so many more bikes and quads in holeshot that it isnt funny, is it worth it, hell yes.