8% is close to what i'd estimate. I would have said 10% based on my avg speed at end of 20 mile "favorite" ride i do often on either gravel bike with disc brakes and 37c gravel tire on rear, 30c cyclocross tire on front: or road bike with 28c road tires and rim brakes. Gravel bike is way more capable and comfortable on gravel. Road bike way faster on paved road. Too bad I usually ride a mix of both!