If we're talking on-road, my record is 57.3mph, but that's stretching the definition of "mountain bike" since it was a mountain bike with 1.25" slicks. That's down Charles Road with a tailwind, a hill
stevemtbr undoubtedly knows well
Off-road, I've topped 40mph momentarily down a steep slope that levels out onto fireroad, but outside of that circumstance, low 30's is about as high as I can get around here. I've been run over by a guy going downhill at an unsafe speed, so I try to keep within my braking envelope when I'm off-roading. Emphasis on "try"
