Originally Posted by LeeG

66lbs is a huge load. I'd suggest 32mm tires as there will be no advantage to 28mm given that load going uphill slowly or downhill fast. If you were a 90kg rider with the increase being muscle as well as bones and your load was 8kg I could see narrower tires having some performance advantage (touring?) as the rider can unweight a lot of the load with their body weight. With 100lbs of bike underneath you you're pretty much picking a line with little time to correct and plowing through whatever shows up in your path. I did a lot of touring at your weight with 8kg of gear on 28mm tires, 6kg on rear, 2kg on front. But once the weight went up another few lbs I was glad I had the larger 32mm rear tire when it came to gravelly roads where I could hear rocks occasionally hitting the rim