Speedometer - Which Gear Needed To Fix Readout?

[sdaughtry]

I upgraded my 1973 240Z, which already had a 5 speed installed, from the stock differential to a R-200 3:90 differential. Now my speedometer is reading at least 10 mph faster than it did before. I found that Courtesy Nissan sells the replacement gear needed to fix the problem - but, I don't know which one I need to buy to get my speedometer reading correctly again. Hoping someone here might know the answer - I did run several searches on the forums (which is how I found the Courtesy Nissan reference)...

[beandip]

Go to datsungarage.com and click on performance tips and scroll down. you will find a ton of good info. There is a list of the different gears and the color and number to teeth I think you need the 18 tooth one if I remember right. Gary

[snag point]

You need a white, 19 tooth cog for the correct reading. http://datsunzgarage.com/engine/. (page 7)

[Nissanman]

How about this?

Sorry, same info, different format:(

[sdaughtry]

You guys/gals are amazing.. thank you for steering me in the right direction!

[Walter Moore]

You need a white, 19 tooth cog for the correct reading. http://datsunzgarage.com/engine/. (page 7)

I am skeptical of that site. They list the 16 tooth gear as being correct for a 3.36:1 differential. My car came with that setup, sort of. The transmission was from a later model 280Z, but with the original 3.36 differential. But when I checked the speedometer and odometer on the interstate both were off. They both indicated around 5% high. 80MPH registered is actually a little over 75, and 10.3 miles registered is a little less than 10.0 miles actual.

I specifically bought 195/65R15 tires so that I would match the OD of the original tires. (175/80R14 = 636mm OD / 195/65R15 = 635mm OD)

Some of this is doubtless the result of "common knowledge" mixing and matching information from different model years. The parts CD does not show a 16 tooth speedometer gear for the "A" series transmission (70-71 models), only a 17 and a 19. The 16 tooth gear doesn't show up until the later models. I wonder if besides needing to match the differential, you also need to consider the year of manufacture of the speedometer.

By the way, the 5 speed that I bought from an 83 280ZX came with an 18 tooth gear, and that car had a 3.90:1 differential. But it also had an 85MPH speedometer, so all bets are off regarding the calibration relationship to an earlier 160MPH speedometer.

Oh, and Courtesy Parts lists the 18 tooth as no longer available. Before you go out on a limb and start buying gears, pull the one that you have and count the teeth.

I put a 17 tooth gear in my car when I changed the transmission, but it has been too cold to drive it on the interstate to check the speedometer calibration. I will let you know what I find out later.

[snag point]

I only know that for my 73 Z, with a 1980 5spd, a 4.11 rearend, and 195 70 14's; used a red 20 tooth cog. My speedometer match my GPS unit readout. I would like to hear from other members. Jon

[beandip]

if it matches the GPS you cant get it better than that. I checked my Wifes new Honda CRV and it is off a couple of miles per. I did the same with my Camry and it is a couple of miles per. fast also. Both are bone stock. Off only .3 in 10 miles to me is not bad for an old Z