No data, eh? Being an insane hoarder of data who records the time, speed, wind speed and air temperature of every commute, I could clearly quantify that moving from the 700x23 Michelin Lithion that my commuter was borrowing from my road bike to the 700x28 (actually 25) Forte GT2/k with Mr. Tuffy didn't just feel slow, it dropped me a good .8-1 mph.