# Speed Sensor Question

**URL:** <https://community.cartalk.com/t/speed-sensor-question/36663>\
**Category:** Maintenance/Repairs\
**Created:** [September 15, 2009, 1:56am UTC](https://community.cartalk.com/t/speed-sensor-question/36663 "2009-09-15T01:56:45Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![TEEJAY2](https://avatars.discourse-cdn.com/v4/letter/t/b782af/32.png) [@TEEJAY2](https://community.cartalk.com/u/TEEJAY2)\
**Post date:** [September 15, 2009, 1:56am UTC](https://community.cartalk.com/t/speed-sensor-question/36663/1 "2009-09-15T01:56:45Z")

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I recently posted about a suspect speed sensor that I thought may be going bad but I had a follow-up question. If a speed sensor is bad wouldn’t it screw up the speedometer reading? This is not happening. The speed seems to be reading true.  
  
  
  
 Thanks guys.

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**Author:** ![Caddyman](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/caddyman/32/23_2.png) [@Caddyman](https://community.cartalk.com/u/Caddyman)\
**Post date:** [September 15, 2009, 2:00am UTC](https://community.cartalk.com/t/speed-sensor-question/36663/2 "2009-09-15T02:00:23Z")

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Year, make, model, and the problem you are trying to cure…

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**Author:** ![TEEJAY2](https://avatars.discourse-cdn.com/v4/letter/t/b782af/32.png) [@TEEJAY2](https://community.cartalk.com/u/TEEJAY2)\
**Post date:** [September 15, 2009, 2:11am UTC](https://community.cartalk.com/t/speed-sensor-question/36663/3 "2009-09-15T02:11:34Z")

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I’ve already posted the question. I just wanted to know if I could rule out a speed sensor going bad if the speedometer is working correctly. ('96 Mazda 626)

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**Author:** ![transman618](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/transman618/32/24_2.png) [@transman618](https://community.cartalk.com/u/transman618)\
**Post date:** [September 15, 2009, 12:09pm UTC](https://community.cartalk.com/t/speed-sensor-question/36663/4 "2009-09-15T12:09:17Z")

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It depends on whether its the input or output speed sensor thats bad. Input sensor reads engine speed, output sensor reads axle speed and is the one which usually controls speedo function.

transman

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**Author:** ![TEEJAY2](https://avatars.discourse-cdn.com/v4/letter/t/b782af/32.png) [@TEEJAY2](https://community.cartalk.com/u/TEEJAY2)\
**Post date:** [September 15, 2009, 7:15pm UTC](https://community.cartalk.com/t/speed-sensor-question/36663/5 "2009-09-15T19:15:33Z")

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Oh, OK. So, I guess the input sensor reads the engine speed and displays that info on the tachometer huh?