So how far off is my speedometer?

[GreenState]

I recently swapped the rear diff in my 4 spd 240z from the stock R180 to a STI R180 with 3.9 gears.

 

The service manual says the 4 spd has a 3.3 final drive.

 

I'm thinking the speedometer is reading a bit faster than I'm actually going. Am I correct, backwards, or just wrong?

 

Thanks.

[78zstyle]

Your speedo is going to read fast. If you have a smart phone you can download a GPS speedo app.

[GreenState]

Your speedo is going to read fast. If you have a smart phone you can download a GPS speedo app.

 

Yep, I can do that... Thanks for the thought, my brain was mired in gear ratio math. That's much easier.

[cockerstar]

Your speedomater should be reading about 18% fast now, assuming you have stock height tires.

You can get the correct plastic cog to go in your transmission to make it read appropriately. It is officially known as a "speedometer pinion gear" in Nissan speak. I believe you'd be looking for the white one (19 teeth).

 

This thread has some good info in it:

http://www.classiczcars.com/forums/thread6997.html

 

The Zhome page also has some good info:

http://www.zhome.com/ZCMnL/tech/R200.htm

 

For those finding this thread while searching, here's which gears you will need for various rear ends to make your speedometer read correctly:

 

3.336 : Yellow (16 tooth)

3.545 : Black (17 tooth)

3.7 : Blue (18 tooth?)

3.9 : White (19 tooth)

4.11 : Red (20 tooth?)

 

 

This eBay listing shows a yellow gear in the picture, but the description lists it as a 19 tooth gear. I would be sure to confirm with the seller that it is in fact a 19 tooth gear. It is listed for about $50+shipping.

 

 

Search tags:

speedometer

plastic cog

speedo drive cable

swapping ratios

spedo reading off

Edited July 3, 2012 by cockerstar