There's a pretty big difference. You have significantly less air volume, so you'll probably have to pump them up more than you're used to, and they will be less comfortable. They should have a smaller contact patch as well. Most people don't use 20's except for special events (read: races). I think it would be worth your time (and possibly money) to exchange them. Others will probably have varying opinions.PinarelloFan said:I bought a pair of tires online and must have not noticed I selected 20mm width..I have never been on anything but 23 mm .. will i notice the 3mm . Are they less safe on corners ???? any help would be great .