I've been riding my Surly Disc Trucker about 300 miles a week to work and just general riding. After 2,000 miles, I thought I would pull out my road race bike and ride it to see to difference. The Surly weighs 32 pounds with fenders, racks and 700X37 tires. The road bike in 19 pounds with 700X23 tires.
I rode the same loops, each about 50 miles, both hills and flats, and compared the results.
The average speed was almost the same, only .4 mph faster on the road bike. I've now got about 300 miles on the road bike and the numbers are the same.
I have to grind up the hills on the race bike, and I spin up in granny gear on the Surly. I ride at a fairly fast pace.
I'm and engineer I get the F=MA equation, coefficients of drag, etc.
Kinds of puts the gram saving mods in perspective.