The new Donnelly 30 mm in the back as it's the biggest that will fit under the brake bridge, even had to file a little off the bottom face with a half-round file. (Bike has short-reach side-pull brakes.) 28 up front. 28 in back worked fine for "good" gravel with clear tracks of hardpack but 30 is a little more comfortable for stoker and still gives smooth rolling on pavement. She likes it a lot. (Well, the chip and seal that passes for pavement here.) The 28 ploughs through most gravel better than I thought it would, enough that we don't exclude gravel roads outright from our local rides any more. Like getting a visa to a new country! 280?-lb team.