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how does the speedometer work?

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YOUR RESEARCHhow does the speedometer work?

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Install larger diameter tires (rear only) i was expecting

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I know there has been other threads covering that the Porsche speedometer seems to read lower than actual speed and again when I installed larger diameter tires (rear only) I was expecting for my actual speed to be more consistent to the speedometer speed.
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how does the speedometer work?

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Leading evidence: 'Install larger diameter tires (rear only) i was expecting' (no explicit attributable resolution; this is a reported action or suggestion). Across 1 matching discussions, The most frequently discussed action is 'Install larger diameter tires (rear only) i was expecting' (1 discussions). For that action, 0 discussions report a resolution and 0 report an unsuccessful attempt. No action has an explicit, attributable resolution report under the current rules. These are forum reports and suggestions, not a confirmed diagnosis.

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Use speed sensors in the rear wheels

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dont quote me on this, but I thought I recall it used the speed sensors in the rear wheels.
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how does the speedometer work?

Replace wiring harness

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Suggestion
The sensor block (that holds speed sensor and brake sensor) crumbled and I lost my speedo so I had to replace the wiring harness that runs from the wheel to the rear circuit panel.
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how does the speedometer work?

Use uniden r3 clocks me at 2 mph

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Suggestion
My GPS-using Uniden R3 clocks me at 2 MPH under the speed on my digital readout - on 18".
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how does the speedometer work?

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Install larger diameter tires (rear only) i was expecting

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I know there has been other threads covering that the Porsche speedometer seems to read lower than actual speed and again when I installed larger diameter tires (rear only) I was expecting for my actual speed to be more consistent to the speedometer speed.
Original post Read captured discussion
how does the speedometer work?

Replace wiring harness

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Suggestion
The sensor block (that holds speed sensor and brake sensor) crumbled and I lost my speedo so I had to replace the wiring harness that runs from the wheel to the rear circuit panel.
Original post Read captured discussion
how does the speedometer work?

Use speed sensors in the rear wheels

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Unsuccessful attempts: 1
Discussions

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Discussions that performed work without a confirmed outcome: 1

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dont quote me on this, but I thought I recall it used the speed sensors in the rear wheels.
Original post Read captured discussion
how does the speedometer work?

Use uniden r3 clocks me at 2 mph

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Discussions that contain suggestions: 1

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Suggestion
My GPS-using Uniden R3 clocks me at 2 MPH under the speed on my digital readout - on 18".
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how does the speedometer work?

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10 eligible posts · 0 unverified posts · 1 discussions match the subject in their title or opening post.

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POST 1 / ORIGINAL POSTEROriginal post

The 996.1 gearbox does not have a speedometer cable, so how does the car know how fast it is going? It has to know what gear you're using in addition to the engine rpm. How does it work? Same question for the odometer.

POST 2Original post

dont quote me on this, but I thought I recall it used the speed sensors in the rear wheels. Not sure though :/ Im curious now as well.

POST 3Original post

My understanding is that manual 996's use one of the ABS encoders (sensors) to calculate vehicle speed. Front left, I read somewhere - but I won't swear to it.

POST 4Original post

Quotes source post: 14750501

This may enplane why my speed did not change (based on the roadside speed signal's) after putting on my new 19" wheels. The rear tires are approx 1" larger than OEM so figured my actual speed would now be same as speedometer speed but my front tires are approx same dia as OEM. C2 Coupe not a C4

POST 5Original post

[Image-only post; no author-written text. Original image references:] https://cimg6.ibsrv.net/gimg/rennlist.com-vbulletin/384x385/speedometer_e845f69790c4d74e2a67f105fd9df82cf000cecf.jpg

POST 6Original post

Great, now can you explain in lamens term what term. 15, term.58 or term.86 S means? I know there has been other threads covering that the Porsche speedometer seems to read lower than actual speed and again when I installed larger diameter tires (rear only) I was expecting for my actual speed to be more consistent to the speedometer speed.

POST 7Original post

Tested mine the other day and it was 4kms under 04' C2 manual with standard size wheels and tyres

POST 8Original post

My GPS-using Uniden R3 clocks me at 2 MPH under the speed on my digital readout - on 18". Thanks for the article, dan, I was going to say I had a rear speed sensor come loose and throw the abs code, but my speedometer was fine. Article explains that!

POST 9Original post

For my 98 C2, the speedo sensor was in the rear (can't remember if it was left or right). The sensor block (that holds speed sensor and brake sensor) crumbled and I lost my speedo so I had to replace the wiring harness that runs from the wheel to the rear circuit panel. Ended up doing all four corner within a month.

POST 10Original post

For what it's worth, it isn't entirely accurate, either. When mine says 70mph, I'm really going 65 - 66. When it says I am going 30, really about 27.

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