JT Sprockets only lists a 15T Fr sprocket for the KZ400, so you'd have to change the rear.
They have several sizes, but you could probably only use a 43T or 44T rear sprocket. stock is 15T Fr & 45T Rr w 530 chain.
The stock gearing with the 45T Rr is 3.000. 44T Rr = 2.933. 43T Rr = 2.867.
Z1Enterprises.com has them, so you could try one, then the other if necessary.
GearingCommander doesn't have a KZ400 in their database, but the 440 would be the closest match. not sure how much help that would be, but you could play around with it. I'd guess the difference in the numbers will be about 10% or thereabouts. They're showing a 6-spd transmission for the 440.
