C5- Speedo accuracy
Thread Starter
Le Mans Master
Joined: Feb 2007
Posts: 9,291
Likes: 1,523
From: Oviedo Florida
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C5- Speedo accuracy
Friends
I posted a thread wondering how fast a C5 coupe was designed to
run at top end ( this day ). I got alot of interesting responces and
I thank you for them. Everyone was commenting about there speeds,
and I wonder how far off GM calculated their speedo's. Average cars
can be off by 5 mph and the higher the speed the more inaccurate
they can become.
Questioin: How accurate do you think your speedo is and do you
think as you get above 100 is it more inaccurate?
Road Pilot
I posted a thread wondering how fast a C5 coupe was designed to
run at top end ( this day ). I got alot of interesting responces and
I thank you for them. Everyone was commenting about there speeds,
and I wonder how far off GM calculated their speedo's. Average cars
can be off by 5 mph and the higher the speed the more inaccurate
they can become.
Questioin: How accurate do you think your speedo is and do you
think as you get above 100 is it more inaccurate?
Road Pilot
Melting Slicks
Joined: May 2000
Posts: 2,974
Likes: 5
From: CenTex Texas
I had a bad speedo a few years ago and can tell you that GM posted a TSB (#83-83-16) stating the following "allowable" variances:
MPH : Variation
20 : +/- 4
45 : +/- 3
65 : +/- 2
As you can see the general trend as speed increases the allowable variance decreases. So i would assume the speedo would be dead-on at 100mph. However over 100mph i bet the speedo will start to lose it's accuracy.
MPH : Variation
20 : +/- 4
45 : +/- 3
65 : +/- 2
As you can see the general trend as speed increases the allowable variance decreases. So i would assume the speedo would be dead-on at 100mph. However over 100mph i bet the speedo will start to lose it's accuracy.
Last edited by 97C5ENVY; Apr 26, 2007 at 08:16 PM.
Tech Contributor
Joined: Oct 1999
Posts: 41,030
Likes: 9,789
From: Charlotte, NC (formerly Endicott, NY)
Accuracy of newer speedometers in cars is not the same as the old pigs we used to drive. In the old cars the speedo was driven by a cable that turned a magnet which induced an electric field for the meter to measure. A lot of things could put them out of calibration. With the new cars the speedometer is still an analog meter but the signal is converted from a digital representation of the speed that the PCM gets from the VSS mounted in the transmission. The speed is calculated from knowing how many times the drive shaft turns per second. A lot more accurate than the spinning magnetic field. Now the analog meter has the same chance of error as the older speedometer analog meter but overall the system is more accurate. However, the HUD just displays the calculated speed and is the most accurate representation you can get depending on your tire size. In most cases it should be dead on.
Bill
Bill
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