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Discussion Starter · #1 ·
I recently bought a '71 with an automatic and 195/70-14 tires, and the speedometer and odometer are both off by more than 10% high. I see in a long-ago thread that one cause of speedometer error could be a weakened spring in the speedo, but am I correct that that wouldn't affect the odometer, so that my problem has another source?

If so, what could be the source of my problem? I also see that speedometer pinion gears are available with different numbers of teeth, so maybe I could fix the problem that way (assuming I don't already have one with the maximum number of teeth), but that still doesn't identify the origin of the problem. Any thoughts?

Thanks.

P.S. Before I fix it, should I get a photo of the speedo at a track day going as fast as I can, just as a memento?
 

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8 / 71 240Z, HLS30-40031, L24-052899, Sunshine Yellow
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If the speedo is reading high and you might ever want to put bigger wheels on it, try that first. Is there any chance the previous owner had bigger wheels on it?
 

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Discussion Starter · #3 ·
I don't think I'll want to put bigger wheels or tires on it, and I'm pretty sure the previous owner did not, but that's a good thought. I guess it's also possible the differential has been replaced, but that's unlikely, too. Nothing like that in the receipts I got with the car.
 

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Discussion Starter · #5 ·
Thanks. Yes, I'll be checking the pinion gear soon, because I'll be changing the transmission oil, and I assume I have the 3.54 rear end. Regardless of my diff. ratio, I guess I can solve my problem by getting a different pinion gear, and then I'll check the diff. ratio sometime when I have the rear of the car in the air. Or I'll just see if the 3.54 makes sense with some tach/mile marker calculations. Carb rebuilds first ...
 

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Discussion Starter · #6 ·
Hmm, my car has the black, 17-tooth pinion gear, which is correct for the 3.54 rear end, I believe. So has the rear end been changed? I'll look again though the receipts.
 

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Originally all 70-74 z's only came with the 3.36 gearing in the diff regardless of transmission style. All 70-73 z's used the smaller R180 diff. You don't have to drain the trans fluid to remove the speedo pinion. All you have to do is jack up the whole car but put the back of the car about a foot or so higher than the front and the trans fluid will go more to the front of the trans. You don't have to remove the cable from the pinion housing either. Just pull the 10mm headed bolt and the pinion keeper and the pinion assembly should slide right out. Look at the color and slide it right back in and secure it. Just so you know the yellow pinions for the 3.36 gears haven't been available for years, and the black ones for the 3.54 gears have been on back order for quite awhile now. Z man of Washington
 

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Discussion Starter · #9 ·
Thanks for the tips, though I'd already checked the pinion. So if my car has a 3.36, then the black pinion should make the speedo read low, not high. Hmm.

UPDATE: I found a service receipt that shows the differential was replaced in 1989 with an "R 180 about a 1980." If that is a 3.54, then the black pinion should be correct. But I suppose it could be anything. I think that explains things. I need to determine what the ratio is, then if I can get the right replacement pinion, I'll be all set.
 

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Thanks for the tips, though I'd already checked the pinion. So if my car has a 3.36, then the black pinion should make the speedo read low, not high. Hmm.

UPDATE: I found a service receipt that shows the differential was replaced in 1989 with an "R 180 about a 1980." If that is a 3.54, then the black pinion should be correct. But I suppose it could be anything. I think that explains things. I need to determine what the ratio is, then if I can get the right replacement pinion, I'll be all set.
My ‘72 service manual confirms my recollection that automatic 240z’s came with a 3:54. Maybe you have a 3:70 diff now. I believe the 1978-79 810 sedan came with an 3:70 R180.
Font Paper product Paper Number Document
 

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Discussion Starter · #11 ·
That’s what I thought, too, though zman says otherwise. Regardless, I think I must have a 3.9 or 4.1 in there now. Higher numerical ratio means fewer turns of wheels for turn of driveshaft, so speedo reads higher. Right?
 

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There are lots of mistakes in the FSM for z cars especially on differential gearing on the US cars. In this case though you're not reading the FSM correctly. For the 3.90 diff gears column it also says "Except USA and Canada" at the top. That means you refer back to the first column which says 3.36 for USA and Canada. ZMOW
 
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