I have managed 62km/h downhill on my MTB with all knobbly tyres... and I felt that I spun out at the 130rpm I was going.
+55 is pretty easy to reach on the short hills here in Medelpad. I went 57,6km/h yesterday in spite of that I am not fit att all and went far from all-out.
However average speed suffer from the hills. I practice spinning, so I am pretty slow uphill, 10-11km/h and lowest gear is pretty common.
I suspect that the downhills will be a blast on the trike... =)
/B
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