Speedometer Calibration Issues

[KMoore68]

Hi,

I have a 1977 911 S Targa and when I was out with the wife tonight for a drive, I was pacing my car with the GPS speed app on my iPhone.

The weird thing is that the speedometer in my car was consistently reading 10 mph up to 60 mph (on the speedometer).

Could the PO have somehow set something in the speedometer to read 10 mph? I believe there is a potentiometer, could that be it?

Kevin

[fanaudical]

Do you have an electronic speedometer? Or is it cable-driven?

Are you saying the speedo was consistently reading 10mph under the "real" speed displayed by your phone?

[KMoore68]

I'm not sure if I have an electronic speedometer or not, I think it is VDO which is electronic, I think.

Yes, the speedo was consistently reading 10mph under the "real" speed on my phone

[fanaudical]

VDO made all the Porsche speedos. Sometime after '75 they switched from mechanical cable driven to electronic (I thought this was part way through the '77 model year). The early electronic speedos have the word "Electronic" on the face.

I don't know what the adjustment looks like for the mechanical units. Here's a previous thread on calibration for the electronic units:

VDO Speedometer calibration how to

[Ctopher]

As far as I know all car manufacturers run slow to actual speed, something like 10-15%. I was told this by North Hollywood Speedo and roughly what I saw in my car. When I sent the speedo in for repair I had them recalibrate based on wheel/tire combination I run....after that the speedo has been dead on to GPS apps.


CTopher

[ClickClickBoom]

Speedometer correction!