700x38 Gravel Kings on our tandem. Fast and comfortable at 60 psi (we are a 310 pound team). We run on a lot of gravel roads without any trouble. Occasionally we go for ATV track type riding and then we switch to 650x48 Gravel King SK's at about 45 psi. We used to have a Comotion Speedster and were limited to 700x28. I had to run them at minimum 100 psi and we didn't ride many dirt roads because even then we were prone to pinch flats on gravel.