Originally Posted by tFUnK

The difference between my road bike and my gravel bike is about 2mph in average speed. Apart from the 38mm knobby tires (vs. 25mm slicks), the gravel bike also has a small handlebar bag, and I tend to ride in baggier clothing when on the gravel bike (though I also do t-shirt rides on the roadie). But otherwise, my gravel bike's geometry isn't too different from my road bike's geometry. I guess i can try some 33mm slicks and ditch the handlebar bag and see how much of that 2mph difference is gained. This is a long-winded way of saying that you might expect up to (but probably not quite) a 2mph improvement in average speed with a set of slicks, all other things equal.