bikes only go as fast as u want them to.
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but na a gsxr1000 would go faster then a dr400 in one gear.when the dr is in 6th lol.
i would think a 400 would be ok for a first bike. alot of sport bike riders start off on 400s.so cant see why a 400 dirt bike would be much differnt.it would probly be easyer to ride then a vfr400.