5 Speed Help

[N268510S]

I own a 68 Datsun 510. It has a re-built L18 with the Hitachi Carb. and a 79 280zx 5 speed. Just started driving the car but when I drive it the speed on the cluster is incorrect. Its runs ten miles over approx. Do I have to change the speedo gear ??? Any help would be apprecitated.

 

[Draker]

You need the one that matches your rear end:

 

from datzenmike

 

[DanielC]

Yes.  You probably need a speedometer pinion with the same number of teeth as the original one, that was in the four speed.  The five may need a different holder for the pinion gear, or you might need a pinion that fits the holder, with the correct number of teeth.

[datzenmike]

You need the one that matches your rear end:

 

from datzenmike

 

 

I believe these are for the earlier Z car so take with a grain of salt. These gears are for whatever tire size was used on them from the factory and it's doubtful the 510 had those.

 

Your speedometer won't be 10 MPH out at all speeds. If it's out 10 MPH at 60 it will be 5 MPH out at 30 and 2.5 MPH at 15 MPH. See what I'm saying? If you say it's 10 MPH out you have to say at what speed. With this info, it can be worked out as a %. Ten MPH fast at 60 would be 16.6% fast.

 

 

 

 

 

 

The following is just an example. First...Remove the speedometer pinion gear and count the teeth and we'll go from there....

 

For argument sake lets assume that it is a 17tooth (Black) gear in the 5 speed. You are 10 MPH fast at 60 or 16.6% fast. Going up in the number of teeth will slow the speedometer. Very, very, roughly each tooth is about 5-6%. A 20 tooth (Red) gear is 20/17=17.6% difference slower. Not bad! that's 1% wrong at 100MPH. 1/2MPH at 50 MPH