speedometer and tach not working on y 1986 300zx turbo. could it be a relay or fuse.if not what should i check

check for the feed from the ignition secondary to the dash~ wires are fragile and can tear easily~

As you mentioned, check fuses first... With so many, if not all of the gauges inoperative, that makes me think electrical issue... If everything in the fuse block passes, check the battery cables and connections. You'd be surprised at the weird stuff a simple bad connection will do without any other issues....

if it is the whole instrument cluster....you have to clean the main ground 1/0 battery cable connection to the chassis ground...wirebrush the lug and the frame where it attaches to assure three hundred amps connection~...this should bring the ground connection back to the dash and instrument cluster~

if the mph gage stop working would this effect the odometer mileage??????