I have a 3.9 R200 diff matched with a 5 speed from an 83 280zx installed in my 70 Z, along with the white pinion gear (19 teeth). I am running the standard 14" wheels with standard size tires. I am finding that my speedo is off by about 10% on the high side and was wondering if I should change to a different pinion gear to have more accuracy. Not sure of any other way to calibrate. Any thoughts?

Thanks!

Edited March 6, 201015 yr by duffman

Define "standard" size tires.

The OEM tires on the early cars were 175/80 14, which are no longer available. The OEM tires had a theoretical OD of 636mm (25 inches). The later cars came with 195/70 14 tires which have a calculated diameter of 629mm and the Tire and Rim Association book shows them at 630mm (24.8 inches). I don't have a T&RA book that still shows the original tire size.

So, if you have 195/70 14 tires on your car, and all other things are right and working correctly you speedometer should read 636/630 times faster than you are actually traveling. Which equals about 1% fast. (You would never notice)

My guess is that either the speedometer is out of calibration, (it is 40 years old...) or something in the system isn't actually the part you believe it to be.

Edit: Have you checked you odometer to see if it is accurate? If the odometer reading is correct but the speedometer is off it is in the speedometer. If both are off by roughly the same amount, poke around on Courtesy Parts to find a 20 or 21 tooth speedometer gear and try that.

Edited March 6, 201015 yr by Walter Moore
Thought of something else.

Thanks, Walter, I should have clarified the "standard" tire size. I am running the 195/70 R14, as you said, and the odometer is off by about the same amount. Essentially, my speedo and odometer are reading about 10% too many MPH and total miles driven. Was thinking about changing the gear to a larger "tooth" gear, not sure if there is anything else I can do.

Unfortunately, the highest number of teeth Courtesy has is the 19 tooth pinion gear (white), which I have and should be correct. Not sure what to do at this point!

A larger diameter tire would help. Something that would give you ~ 25" in tire diameter. But then there's the "rub" factor.....

A larger diameter tire would help. Something that would give you ~ 25" in tire diameter. But then there's the "rub" factor.....

, i'm picturing OP putting "DUBS" on his z, and trying to explain to people it was a necessary repair.