How to correct your speedometer (and odometer) after changing your differential or transmission.

This writeup is meant to assist someone who is trying to correct their speedometer after changing their differential to a new final drive ratio, or after changing their transmission.Correcting your speedometer after putting different-sized tires on your car is beyond the scope of this writeup, and therefore will not be discussed except for this note: With a little math, it may be possible to use this writeup to get your speedometer a little closer, if not precise, depending on what tire size you've chosen.

How the speedometer works:
The speedometer (and odometer) are driven via a cable by a pinion gear in the rear of the transmission, right side. This gear meshes with the transmission's output shaft directly. The pinion gear can have anywhere from 16 to 23 teeth, depending on the differential you are using. All that is needed to make sure your speedometer is accurate, is to install a pinion gear that matches your differential (again, this is assuming you have the proper tire size). Pinion gears are also color-coded for easy identification. Here is a listing of available pinion gears, their colors, the differentials that they are meant to be used with, and their nissan part number. Important note: If you use a pinion with 20 or more teeth you must replace the sleeve that holds the pinion gear. The sleeves for 20+ tooth pinions are offset to allow for the larger size of the pinions. Not using the proper sleeve will destroy the pinion gear and possibly the drive gear inside your transmission.

Examples:

• You bought an aftermarket 4.11 differential cause you are shooting for 12's on stock fuel. You don't mind so much the high-revving all the time, but the fact that your speedometer is reading really high is starting to bother you, plus it's causing your odometer to tick off the miles too quickly. All you need to do is order the 20-tooth pinion gear, and install it into your transmission. Viola! your speedo is working properly again.
• You recently installed a T5 tranny (84-86 Turbo) in your 86 NA car cause your NA tranny blew up. Since the 84-86 turbos came with a 3.54 differential, the pinion gear in your new tranny is the 17 tooth. Since your 86 NA differential is the 3.7, you need to put an 18-tooth pinion gear into your new tranny, which can be convieniently found already installed in your old tranny.

Caveats:

• Some models may have the wrong diff for their model year, for example, it's been reported that early production date 87 turbos might still have the differential that came with the 84-86 turbos. To be sure of what differential your car came with originally, look at the model plate under the hood. (The ID plate is wrong for 88-89 models, see the differential page). If your car originally came with a 3.54 diff, it'll say RC35 somewhere, 3.7 diff will say RC37, and a 3.9 diff will say RC39
• 21, 22, and 23 tooth gears are nowhere to be found. They appear in the z31.com archives, but their part numbers are elusive. Perhaps they are used in big nissan trucks, but I sure can't find them.

By WindigoZ 04/01/03