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You didn't say what kind you had. Some of the big 2-inch wide slicks are very thick and it makes for drag. They are also heavy. A 1 or even a 1.4 would probably make a difference you could feel right away.

I disagree with these guys on the tires. I have had 23mm tires on a road bike and they just don't ride that harsh if you stay off the bigger bumps.

Instead of going 2mph faster, you can go the same speed with less fatigue. If the difference is really 2mph, then it's a pretty major decrease in rolling resistance, even if it may not boil down to a huge difference in time.