Ok, this is the question, 280zxt, running P225 / 50R / 15s

Stock tire from FSM states, P205 / 60R / 15s

I have a late model 5 speed ( one Tang ),

R200 Diff, suppose to be 3.54, came with all Turbos, auto or Manual

I have a black cog 17 tooth.

My speedo is running 10 mph to fast, so says my GPS.
And I dont think the tire size would cause that much difference ?

I also have a White cog, 19 tooth, so if it has more teeth it should take more time to go 1 revolution, which should slow down the cable, and slow down the speedo.
Im not sure If this car has a 3.54, and since it has no leaks in the diff, I dont want to open it up to see.
Will the White cog slow down the speedo, Am I right or way off base.
I could live with it this way, but I hate doing math while Im driving.

 

Still no Takers , Yea I know the Math isnt easy, thats why I posted it.

Probably going to get under the darn thing and install the White Cog to see what happens, and I wont be posting the results, you will just have to guess.
Joking. I might post, I might not, since Im the only one who answers my posts, dosent make much difference does it. Joking I will post, maybe

 

I have read all of your posts and they are fairly clearly written and interesting. Thank you.

About your speedometer error, you remember that a larger (or smaller) tire diameter increases (or decreases) the wheel circumference 3.14 units of circumference to one unit of diameter. Since your speedometer is counting wheel rotations and just assumes the distance traveled per rotation in order to calculate your speed, an error of 10% is possible. A larger tire travels farther with each rotation and your speedometer is assuming distance traveled per rotation of its original-design (OEM) tires - which are gone now and were likely a different circumference than your present tires.

On a larger diameter tire you make fewer tire rotations at a given speed so your speedometer indicates a slower speed than you are actually traveling. If your new tires have a circumference 10% larger or smaller than the Nissan OEM factory tires your Z was designed for, your speedometer reads 10% slower or faster, respectively. You move at 60mph, speedometer says 54 (larger than OEM tires) or 66 (smaller tires). Easier to correct that in your head (fun) than to re-engineer and re-build your speedometer, (aargh) yes? Or its moving parts are worn and tired, given age and use.

Neither of my '86 Z31s measured my speed really accurately. I just knew that it was less than 100mph in city traffic and anybody's guess on the highway. Speed-control officials always have a surprise for me in our frequent meetings at the side of the road. They definitely know my speed - and wrote it down, too. Cool.

In an earlier post, I think you mentioned a drop-off in general site activity here over your years here. The oldest truth on the internet is that you can offend and insult (humiliate - at a safe distance) people you have never met and avoid the quick punch in the face you would get in person. This site generally does not do much when a regular spouts off at a new poster for not knowing the answer to the question the poster came here to get. I don't believe it is a crime, or stupid, not to know a fuse from a relay. We all learn in the same way and have the time to help each other - the idea behind the early (disappeared) internet. You clearly get that. Just ignore the bozos and keep your visits frequent.

You are a welcome contributor to this site and add a good element.

Please post a pic of your Z. Cheers and thanks again.

 

Thanks ZXGUY,
All made since, a 10% difference due to running smaller tires, which came with the car 12 years ago, and will be going back to stock when needed, 10% I can understand, but Im reading 40 mph when Im going 30, thats a what 30% difference,spedo Is steady, yes its old. Guess the only way is change the cog to white.
Thanks so much for the help, I posted on this board probably 400 to 500 posts, probably 10 years ago. I like helping people when I know or think I know the answer, everybody just says, search. But I know the search engine isnt what it should be, cause Im always searching. Bring back the old internet.
Thanks

 

OK, a 33% speed error is off the charts. That speedometer is no longer giving you useful information. You might find a working replacement from a junked 280Z at a recycler near you somewhere. This is a long shot but check out Car-Part.com--Used Auto Parts Market for a start, there are others. Since a recycler is more likely to remove and shelve starters, radios, etc. from the Z, the gauge is probably still in the dash there on the lot. If you can find a Z on a lot near you, it could be a source for many other parts, too. Call their 800 number and ask if they still have the Z there. Worth a try. When the Chinese start making a universal speedometer replacement kit, cool, but the gauge won't match any OEM gauge in any dash - which will look klunky as hell.

Sorry I don't have any input on your transmission. I have never dealt with AT in a Z.

Keep up the good work.

 

Yea, I thought that was a little high, good news is I have another Spedo, in waiting.

I do have a Manual transmission, 5 speed later type, 1 Tang type, I switched out the AT first thing after buying the car.

Yes I haunted junk yards for 2 or 3 years when I first got the car, I have alot of extra parts. Bad thing is there are no more Zs in the yards anymore around here. One of the largest one, informs me when a Zs comes into the yard, havent got a email from them for years. Do get them for other cars I own tho. Believe they are just to old.

Guess I will pull the cluster out one more time, this will be probably the 30th time and change out spedo.

 

Follow up,
Thee short cable going to the speedometer broke again, think Im just going to give up on
the speedometer, at least for now, tired of removing the cluster, just use my GPS, some things
Datsun/Nissan did right some not.