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Can speedometer degrade

1041 Views 10 Replies 8 Participants Last post by  Astro Bob
I've posted before that my mileage in the 83 just suddenly dropped. Now the son has had it for about 3 years. When I had it, I got good mileage AND I know the speedometer was correct. He got good mileage til about a year ago when all of a sudden it went to crap. We've had a lot of theories which I won't go into. Recently he was going back with his friend in a new Nissan. When they got there, Matt said they were averaging 80+ while the Z was registering 74. So next trip we confirmed this with the Garmin. Oddly, following the mrs thru town, we both pegged 40 mph.

So, is it possible for the speedometer to run into resistance (I suppose the cable itself hopefully not the gauge). Is there a gear in the tranny that is replaceable and would correct this? If He is running slow, that would account for a drop in miles and thus mpg.
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an 83 speedo won't register more than 85 mph thanks to jimmy carter and our nanny state government. the odometer should work unless something broke back by the tranny gear. If you don't track the miles and gas over a few hundred miles any info you get is useless. One tank can give you any number you want. but data for a 1000 miles should give you a good gage of what the mpg is on your car. read your own post replies.
That didn't really answer the question. Perhaps I give too much info. I got to thinking later that maybe I need to pull those cables out, make sure they are greased and look for kinks.
A question, is the odometer mileage different than a different car on the same trip? I did speedometer work about 15 years ago on the ZX and this is how I remember it worked.

The odometer is gear driven off the cable, if the odometer mileage is different than the other car I would think the problem would be in the cable or the speedo gear at the tranny.

The speedo is magnetically driven with no physical contact. If the speedo needle pivot point was worn or dirty it would be off, but the odometer would be correct. Also, maybe the magnet strength changes with age? Have it calibrated with the 60mph= xxxxrpm's stamped on the speedometer, I wonder if they just adjust(bend) the spinning coupling gap.

Also, cable issues usually have the bouncing needle issue at slow speeds.
Good luck!

PS palladin, I still have not found a test report where the 84 turbo is faster that a 280ZX turbo. Butt-dyno test don't count, if they did my car would be the fastest car ever ;D
I never really thought about the odometer giving erroneous readings.On my 1982 280ZX coupe the speed reads 4.5MPH less than my actual speed, while on my 1982 280ZX 2+2 (both turbos) the speed reading is 6MPH faster than I am actually travelling.
it has to be an age isssue.

WI, all I remember is that 84 wasmuch more respnsive and you didn't have to kick in tjhe turbo all the tyime.
The plastic gear (color coded to match your diff gears) is one of the "usual suspects" since they get worn and as a result turn in a smaller circle.
I wondered if these had those plastic gears. The extra tranny I had apparently does not have one. Available thru nissan I suppose?
Yes, I've seen them in the Nismo catalog. Color coded for the diffy gears. You could probably just get them through the dealer.
Bad magnet in the speedo gauge,Happened to me,replaced cable and the tranny speedo gear with no luck,still under reading for about 12 miles/hr.
Every thing went back to normal after replacing the gauge.
To check this, try using a portable GPS navigator. I was sort of forced to use one when I bought my 79 ZX, as the speedo did not work at all. (broken cables, six inches from the tranny). Mine was an older model Garmin GPS3+, which had a special window that I could configure to show the data I needed. I had one window pane show MPH, and another kept track of miles driven since a reset, and another with total miles driven. I feel certain that using this kept me from getting several speeding tickets!

Most modern GPS navigators are mostly a moving map display, but most of them can be configured to show MPH and keep track of trip mileage. That can let you check the calibration of your speed and your miles driven, plus calculate an accurate MPG.
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