Originally Posted by gregf83

I saw 3-4 kph difference between commuting on studded winter tires vs Conti GP4000s. You're unlikely to see anything close to that.

For a rough estimate it's easily possible to save 15-20W with decent tires. Your power to go 18mph is likely in the 150-200W range so you might get an extra 10% power applied to moving you forward. That will make you roughly 3% faster or 18.5mph instead of 18.

What kind of wheels/tires are you riding now?