Want to buy 350z nismo 260 -320 kph speedometre

if not you should be able to buy the wiring kit for it to do so , look on ebay that is the best place to find one and also the best price , good luck

I would have it done there. They can give the car a good German tuneup and get it Autobahn ready.

could be bad speed sensor.

Have the speedometer panel tested for errors, and check the fuse box for correct operation's.

If it's digital there's a button somewhere I'm sure. However if it's a printed read out, then the cluster has to be changed over...however it should also read both. This is a common issue for service men bringing vehicles back from Europe as did I in 1998. Hope this helps !

ground not making good connection to frame by gas tank sending unit ground

Your warranty will only cover it if it is installed when the vehicle is purchased new.

install a free flowing aftermarket air intake kit, like one from K&N corp.

Yeah, i had this problem once... replace the speedometer and the whole system there.

Oh, i had this problem! you need to replace the clock i don't know the name but you need to replace the thing that shows you in which speed you are and the time and things.