[pandaman]

Gravel bikes are much more comfortable and fast than road bikes on unpaved roads, which is what they are designed for. Wider wheels will add more weight and more angular momentum, making it harder to climb and harder to steer so you might need to replace the drop bar with the MTB flat bar and without the drops you can move the saddle into a more comfortable position as well... ending with a hardtail w/o the suspension fork.

[bluecalm]

Fast MTB tires are like 100g heavier than current crop of gravel tires. Not a big penalty as they roll much faster and are have much better grip.

[pandaman]

40 mm tires are ~500g, what kind of 600g MTB tires you have in mind?

[l1tany11]

Kenda booster & rush. Continental race king. Maxxis Aspen & Aspen ST. Schwalbe RRs, Vittoria Mezcal, S-works Renegade & fast track, Pirelli Scorpion. Basically every MTB tire manufacturer makes an XC race tire in the 600g range. Some are below 600g, some are above 600g (but below 700g). Just look at the list on the website this thread is under.

[pandaman]

But these do not show amazing rolling resistance on that site either. Only Race King approaches top RR numbers of the gravel tires. There are plenty of sub 400g gravel tires with less RR than these. As far as I understood my correspondent, he predicts that riders will add angular momentum (when and MTB riders already complain about steering on gravel bikes) and, apparently, won't win any rolling resistance. What for? Riding at even lower pressure? To do what?