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Discussion Starter · #1 ·
I swapped my oem 4 spd in my 75 for a 81, 5spd (280b). I did like it said on a L28 page I found (www.geocities.com/motorcity/2824/engine.html) and put my original cog from my old tranny into my new tranny but now my speedo is off by about 7 mph (used a gps to check speed). The faster I go the worse it is. I used both the cogs I have, the one that came with the 5spd and the one from my old 4 spd. The one from the 4 speed is closest to being right. Any idea's on what I can do to make my speedo accurate? My tires are the same height as stock. Any help would be appreceiated
Rob,
 

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Discussion Starter · #2 ·
Did you change the diff out that is what causes the speedo to get off. You need the cog that matches the diff you have. Sam 75 280z
 

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Run a mathematical calculation on your gearing. If you know your tire diameter or circumference and the differential ratio you can calculate theoretical speed in any gear at any engine rpm. Then you take it out on the road and see how close the speedo is to theoretical. If you would like, I can redo the formula and post it for everyone.
BTW - you didn't mention if it was reading fast or slow or what size your tires actually are.
195/70-14, 205/60-15, 215/50-16. For any of these wheel diameters and profiles if your width is smaller you will go slower than speedo indicates and faster if your width is more.
Phantom
 

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Discussion Starter · #5 ·
MORE info on My speedo's off now

When my speed indicates 82mph my GPS indicates i'm doing 75mph. My tires are 225/60/14 Yokahama 509's (not that brand should make a differance).

Phantom, I can understand tire height making the speedo change but how or why would the width change it?
 

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RE: MORE info on My speedo's off now

I calculated your tire size vs stock, height difference is
24.6 (yours) and 25.5 (stock assuming a 195/75-14), divide stock by yours you get 3.6 % error. 75 * 1.036 is 77.7 mph. Stock for your car may actually be taller than what I used which would explain why your reading 82 and going 75.... Its very possible that the error has been there since the day you put the tires on and never noticed till you compared to the GPS after changing trannies.... Hope this helps to explain.
Steve
 

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RE: Thank's Steve

Steve,
Tire diameter is a function of its width and profile. IE: distance from the wheel rim to the tread is the width in mm times the profile. Example: Your tires are 225mm wide therefore the distance from your wheel to the tread should be about 225x.60=135mm or about 5 3/8".
That would roughly make your tire diameter 14" plus 5 3/8" plus 5 3/8" equals 24 3/4". The actual, from the book is 24.65". The stock 195/70-14's are about 24.93".
With that difference, if your speedo says 60 then your GPS should say 60 x 24.65/24.93=59.3. Looks like you still have a little more calibration error to find.
Phantom.

PS: My tire diameters are taken from an article in an issue of Sports Cars of the World magazine from sometime in the late 80's.
 

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Discussion Starter · #9 ·
<b>RE: Thank's Steve</b>

OK, if I recall the stock tires are 195/75-14. I could be wrong but the 75 vintage cars had tall tires. I am fully aware on how to calculate tire height vs. width ratio's and I stand by my original error. Less someone has documentation on the 75 Z having the original size tire other than 195/75-14.
 
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