Speedo error not linear
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Joined: Mar 2005
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Speedo error not linear
This one really frazzles me.On my 82 The speedo starts off slow, like does'nt even move till 15mph,then it play's catch up till about 45mph.At 50mph it is right on matches gps and other cars I check against.From 50 to 85 the max the speedo goes it is 100% perfect,slow down speed up it reacts perfect and matchs gps.If it's the wrong gear it would be consistant,yet under 45 the error is way off ( like it shows 29mph when the car is going 36),But is always dead on above 45/50 mph? any ideas?
Team Owner
Joined: Jul 1999
Posts: 65,492
Likes: 230
From: Orange Park Florida
MY '72 stock speedo is right on till about 80mph, which is 78 in reality....I believe GPS.....
on ~40 y/o cars, all sorts of crap has gone on over the decades, and who is to say the instrumentation was all that good when NEW???
I KNOW for a fact that I thought I was going 150 mph and was RONG, because of the TACH being rong and proven so by me Tektronics scope....and then the mechanical speedo with the supposed gear tooth count by the various sites with tire diameter, rear end ratio, and final drive ratio via tranny.....and those are all BS.....
off by over 10% at best.....
and even when correcting the gear tooth counts for all that, the speedo insisted on registering a HUGE over MPH at anything better than 80 mph.....for instance 100 mph indication was really only 94 mph.....
methinks it's built into the gauges from the factory, no matter what....
on ~40 y/o cars, all sorts of crap has gone on over the decades, and who is to say the instrumentation was all that good when NEW???
I KNOW for a fact that I thought I was going 150 mph and was RONG, because of the TACH being rong and proven so by me Tektronics scope....and then the mechanical speedo with the supposed gear tooth count by the various sites with tire diameter, rear end ratio, and final drive ratio via tranny.....and those are all BS.....
off by over 10% at best.....
and even when correcting the gear tooth counts for all that, the speedo insisted on registering a HUGE over MPH at anything better than 80 mph.....for instance 100 mph indication was really only 94 mph.....
methinks it's built into the gauges from the factory, no matter what....
Limping across the line
Joined: Oct 2002
Posts: 5,299
Likes: 109
From: Peoria Az
Once you dismember the speedo head to see how they really work you can understand how they are erroneous.
Basically there is a mechanical connection from the trans to the back of the speedo head. Inside the speedo head the cable is spinning a set of magnets. The magnets use magnetism to then pull a pickup tab that is connected to the needle.
Basically there is a mechanical connection from the trans to the back of the speedo head. Inside the speedo head the cable is spinning a set of magnets. The magnets use magnetism to then pull a pickup tab that is connected to the needle.
