Originally Posted by
chas58
That said, I did to an experiment once - taking a 18lb gravel and 28lb mountain bike on a 3 hour gravel ride. Both had excellent tires, and were pretty close in time for the first 1.5 hours. But, after the 2 hour mark I was just beat carrying the extra weight up our rolling hills, and my time fell off dramatically. I really struggled on the 28lb bike for the last 30 minutes. So, I know I have better endurance on a lighter bike - even if I'm running a slower endurance pace.
How's the position? Sitting upright on a MTB is a ton more aero drag than even a semi-upright endurance position. Which will likely make more of a difference than weight. So will rolling resistance on 35mm semi slicks vs 2" knobbies.
Mostly I ride hardpack, and I can do it on 26mm gravel kings. But 40, 45mm tires give me a lot more flexibility about choosing lines and let me carry more speed because I'm not dancing around rocks and ruts.