NOS (new old stock) speedometer pinion gear full assembly, including the sleeve, seals, and the retaining pin. It has a 20-teeth gear & measures about 3 1/16" in total length.  17, 18 and 19 teeth gear assemblies are listed separately. Please make sure you compare this assembly and gear to your existing assembly. Some Datsun transmission use a different longer gear. Below is a guide on how to choose the proper speedometer pinion for your car; the number of teeth you need is based on the rear differential gear ratio. 

Applicable models: Datsun 240z, 260z, 280z, 280zx (1970-1983). Nissan 300ZX (1984-1989).  Also, Datsun 510 (1968-1973). Datsun 200SX (from 07/81-07/83)

Gear Ratios
16 Teeth Gear: 3.36 Ratio
17 Teeth Gear: 3.54 Ratio
18 Teeth Gear: 3.70 Ratio
19 Teeth Gear: 3.90 Ratio

Datsun Models & Corresponding Diff Ratios (from factory)
240Z 70-73 MT (3.36) (R180)
240Z 70-73 MT 5spd. (3.90) (R180) - non US
240Z 71-73 Auto (3.54) (R180)

260Z 74 MT (3.36) (R180)
260Z 74 Auto (3.54) (R180)

280Z 75-76 (3.54) (R200)
280Z 77-78 MT (3.54) (R200)
280Z 77-78 Auto (3.54) (R180)

280ZX 79 MT,4 Spd,2 Seater (3.36) (R180)
280ZX 79 MT,5 Spd,2 Seater (3.36) (R180)
280ZX 79 All With Automatic(3.54) (R180)
280ZX 79 MT,4 Spd,4 Seater (3.36) (R200)
280ZX 79 MT,5 Spd,4 Seater (3.70) (R200)
280ZX-GL 79 (3.70) (R200)
280ZX 80 MT,2 Seater (3.54) (R180)
280ZX 80 Automatic 2/4Seat (3.54) (R180)
280ZX 80 MT,4 Seater (3.90) (R200)
280ZX-GL 80 (3.90) (R200)
280ZX 81-83 Automatic all (3.54) (R180)
280ZX 81-83 Turbo - all (3.54) (R200)
280ZX 81-83 MT (3.90) (R200)

Datsun 510: 1968 (3.70) (R160)
Datsun 510: 1969-1973 (3.90) (R160)