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Discussion Starter · #1 ·
I've got a 79 ZX..with a five spped.
When I bought it nine years ago, I noticed the speedo read maybe 8-10MPH high.
The car is a "GL" model, as are most of them, which according to various literature, has a 3.7 rear end.
A couple years back, I put 60-series tires on it, actually a little smaller in diameter than the originals (215-60-14 vs. 195-70-14), and now the speedo reads 12-15 mph high!, especially at higher speeds.

Two questions..
One..do you think the factory put in the wrong "gear" for the speedo, like one that should have gone on an automatic with a different rear end ratio?
Two, anyone know how difficult it would be to change to a more accurate "gear", and which one would I need?
 

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Discussion Starter · #2 ·
Hey man,
I am having the same problem. And I too have the 60 series tire. I have 225-60-14 rear 215-60-14 front. I don't undastand it. I always leave before mom and dad and they always catch up, so we checked it and mine is about 10 slower. But i never know, i'm afraid i'm going to get a ticket cuz who knows, one day it could be right. I might actually be doing 75!

simon
 

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Discussion Starter · #3 ·
<b>RE: Speedometer readings</b>

The speedos are not all that accurate to begin with and changing around tire sizes usually complicates the matter. The speedos are "more" accurate at slower speeds and slowly become more inaccurate the faster you go. So in town your speedo will be closer to accurate when compared to blazing along on the highway.
You can change the speedo gear in the tranny and is quite easy actually, though I can't recall the gear set numbers you need. (They are actually colour coded.)
I believe I saw the info you need at a web site that is about "How to swap in a 280 motor, 5spd transmission, and R200 diff" The actual site name eludes me right now.
Maybe someone out there can provide a link?

ZR8ED.
 

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Mine is pretty much always about 20% off. This is my fault. The factory put in the right speedo gear but I changed the gearing when I put on the 225.50X15 tires and the 3.9:1 differential. I think its Brian Whites Datsun Garage site that has the different colors for the gears. I just do the math on the fly. In fact it is kind of funny how I figured out that it was 20% off. When I got to California I noticed that when I was driving my usual no more than 10mph over the maniac California drivers were crawling all over me to pass me. Then my wife followed me to the to the base one day in the 4Runner and said "you drive too slow" I was doing an indicated 10 over.
 
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