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Old 04-09-2019, 05:44 PM   #1
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How does the Boxster determine it's speed?

As the title asks. How does the 986 Boxster determine it's speed? I did a internet search and from what I could tell is it does it from the ABS sensor? Then I read a post that the systems takes into account all four ABS sensor speeds and gives out an average.

So I'm really not sure about this. Can someone enlighten me please on how the 986 Boxster determines it's speed?
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Old 04-10-2019, 06:48 PM   #2
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I know that changing the tire radius will cause the speedometer to report an incorrect speed, well the factory set speed. I'm not sure if it's being based off the front tires or rear tires?

Anyone know?

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Old 04-10-2019, 08:42 PM   #3
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By my mood and right foot!
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Old 04-11-2019, 09:41 AM   #4
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Originally Posted by Boxstard View Post
By my mood and right foot!
Yeah my mood and right foot also affects the speed my Boxster is traveling at.

How does the 986 Boxster determine the speed the car is traveling at?
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Old 04-13-2019, 07:44 AM   #5
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I'm pretty sure it uses the right rear wheel speed sensor for the VSS signal.

Easy way to test, jack the car up, turn the igntition to run and spin the wheels. See which ones move the needle on the dash.
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Old 04-15-2019, 02:51 AM   #6
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I'm pretty sure it uses the right rear wheel speed sensor for the VSS signal.

Easy way to test, jack the car up, turn the igntition to run and spin the wheels. See which ones move the needle on the dash.
Jack up the car and see which wheel it reads the speed from? So nobody here knows for sure?
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Old 04-17-2019, 08:39 AM   #7
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Jack up the car and see which wheel it reads the speed from? So nobody here knows for sure?
Soooo, why does it matter? I can't say for sure, and I really don't care.
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Old 04-17-2019, 08:53 AM   #8
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Soooo, why does it matter? I can't say for sure, and I really don't care.
Why does what matter? The fact that people on the forums don't know the answer to this or that I want to know how the Boxster determines it's speed?

It matters to me because I am going to change the diameter of my tires and I want to calculate how far off my speed will be if the car judges the speed from the tire rotation speed.

Thanks for not answering the question and being a jackass for nothing.
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Old 04-17-2019, 09:19 AM   #9
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Soooo, why does it matter? I can't say for sure, and I really don't care.
And the winner of "most helpful and relevant post of the week" goes to......

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Old 04-18-2019, 01:52 PM   #10
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Originally Posted by KRAM36 View Post
Why does what matter? The fact that people on the forums don't know the answer to this or that I want to know how the Boxster determines it's speed?

It matters to me because I am going to change the diameter of my tires and I want to calculate how far off my speed will be if the car judges the speed from the tire rotation speed.

Thanks for not answering the question and being a jackass for nothing.
You are quite welcome.

I think you will find that I am generally not a jackass, and go quite far out of my way to help people find answers that I often have no other use for. In this case it would have been nice to know the scenario before calling me a jackass for not helping. I don't like spending a bunch of time because someone was "just wondering", hence my comment. This actually piqued my interest last night and I even spent a half an hour of my time looking up exactly how it calculates speed. I looked in the Bentley, the factory (996) 911 Technical Introduction Service Information manual and called a friend that I thought would know as he sets uses this info in 911 and Boxster race cars. Why would such a jackass go to this trouble? Maybe he likes abuse.

Yes, of course the system uses wheel speed to calculate vehicle speed. That is the only way it could. Each "tone wheel" that the wheel speed sensors read has 48 teeth that is read by the ABS module. The ABS module then divides this down by eight and sends it to the appropriate module. The real question is does it use one sensor, does it somehow derive it from all of the sensors, or? Well, the answer, as far as I can tell is a bit tricky. It appears that there are two speed signals sent out. One from the right rear, and one from the left rear. The key is that it seems that the speed is derived from the rear wheels, not the front, so hopefully that answer is good enough. But hey, I am just a jackass.
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Old 04-18-2019, 02:10 PM   #11
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Originally Posted by KRAM36 View Post
Why does what matter? The fact that people on the forums don't know the answer to this or that I want to know how the Boxster determines it's speed?

It matters to me because I am going to change the diameter of my tires and I want to calculate how far off my speed will be if the car judges the speed from the tire rotation speed.

Thanks for not answering the question and being a jackass for nothing.
just load waze or a speedometer app on your phone and you can easily see what your speed is relative to the speedo

And Qmulus is far from a jackass
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Old 04-18-2019, 03:01 PM   #12
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Originally Posted by JayG View Post
just load waze or a speedometer app on your phone and you can easily see what your speed is relative to the speedo

And Qmulus is far from a jackass
I don't put apps on my phone. Google gets enough info off me with just the crap that's loaded into Android phones. I want to figure the percentage of difference on my own and I need to know how the Boxster 986 determines it's speed. Front tire rotation speed, rear tire rotation speed or neither?

In this case he was being a jackass.
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Old 04-18-2019, 05:19 PM   #13
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Originally Posted by KRAM36 View Post
I don't put apps on my phone. Google gets enough info off me with just the crap that's loaded into Android phones. I want to figure the percentage of difference on my own and I need to know how the Boxster 986 determines it's speed. Front tire rotation speed, rear tire rotation speed or neither?

In this case he was being a jackass.
I guess this jackass shouldn't bring up the fact that the speedo reads high by design either.

It used to be that speedos got their signal off the output of the transmission, and gears were used to calibrate the speedo to tire size. Now the cluster gets a signal from the ABS and uses a calibration factor (like pulses per mile) to calculate speed.

In the end, it really doesn't matter how the Boxster calculates its speed. If you assume it is accurate now, which it really isn't going back to the first part of this post, all you have to do is calculate the percentage difference in circumference of the new rear tires compared to the originals, just like you would with any speedo.

Now if you want to actually fix the speedo so it reads right, you have to adjust the calibration factor in the cluster to match the tires being used. On some vehicles that can easily be done with diagnostics. Not quite as easy on these.

But what do I know; I'm just a jackass.
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Last edited by Qmulus; 04-18-2019 at 05:21 PM.
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Old 04-18-2019, 07:05 PM   #14
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Originally Posted by Qmulus View Post
I guess this jackass shouldn't bring up the fact that the speedo reads high by design either.

It used to be that speedos got their signal off the output of the transmission, and gears were used to calibrate the speedo to tire size. Now the cluster gets a signal from the ABS and uses a calibration factor (like pulses per mile) to calculate speed.

In the end, it really doesn't matter how the Boxster calculates its speed. If you assume it is accurate now, which it really isn't going back to the first part of this post, all you have to do is calculate the percentage difference in circumference of the new rear tires compared to the originals, just like you would with any speedo.

Now if you want to actually fix the speedo so it reads right, you have to adjust the calibration factor in the cluster to match the tires being used. On some vehicles that can easily be done with diagnostics. Not quite as easy on these.

But what do I know; I'm just a jackass.
Of course I know the speedometer is not showing the actual speed, even from the factory, especially with German cars because European law (ECE-R39) says speedometers cannot show speeds less than the actual speed, and they must never show more than the 110 per cent of actual speed plus 4 km/h. So, under those rules, a car could be moving at 100 km/h, but the speedometer could legally display as high as 114 km/h.

Yes I know cars used to determine the speed from the transmission, some possibly still do. I have owned many GM vehicles that I could change out the gear attached to the cable that went from the transmission to the speedometer. I put a HiPo 327 into a Chevy S-10 that had a Muncie 4 speed and way lower profile tires and I changed the gears on that. Heck I eventually put in a full digital dash into that truck.

Right now I'm on my winter tires I have had my car's speed read by actual radar and the actual speed is off about 7% on the high side with these tires. So I know when I'm doing 38 MPH on the speedometer it's actually just over 35 MPH, when the speedometer reeds 65 MPH the actual speed is just over 60 MPH. I know I can drive at 70 MPH on the speedometer and be right at 65 MPH. So instead of messing with the radar readings again and knowing how the Boxster 986 determines it's speed, I can just calculate it out now that I know how my speedometer is actually reading per the circumference of the tire.

I'm buying a set of summer wheels and tires so I don't have to go get the tires changed out at the change of season. If it's the rear, tire it won't be a big change, but enough I should factor it in. My winter rear tires are 812 revolutions per mile. The summer wheel/tire setup 817 revolutions per mile, both front and rear. Unlike the factory setup that had the front tires rotating 38 more times per mile then the rear, which makes the car twitchy in the front.

Anyways, about the jackass part, yes your were being a jackass. After you got called out on it and realized it was true, you decided to redeem yourself and come back with some actual useful information. I'm old enough to know how people work, you're not pulling the wool over my eyes. BTW: I didn't say you're a jackass 24/7, I said you replied like a jackass, which is true.

Now I'm glad you responded like you did and got called out on it as I'm finally getting some useful information in this thread.
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Old 04-18-2019, 07:46 PM   #15
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KRAM36, you are the one being a jackass. Why don't you just move on?

You can't recognize it when someone is trying to be helpful, but needed clarification on what you were asking for. I wasn't sure just what you were looking for either.

And you continue to bury yourself.

Go ahead and blast me, it won't be me that looks like a fool.
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Old 04-18-2019, 07:50 PM   #16
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KRAM36, you are the one being a jackass. Why don't you just move on?

You can't recognize it when someone is trying to be helpful, but needed clarification on what you were asking for. I wasn't sure just what you were looking for either.

And you continue to bury yourself.

Go ahead and blast me, it won't be me that looks like a fool.
You already made yourself look like a fool with this post, no need for me to blast you. Why don't you just move on?
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Old 04-18-2019, 08:40 PM   #17
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Originally Posted by KRAM36 View Post
I don't put apps on my phone. Google gets enough info off me with just the crap that's loaded into Android phones. I want to figure the percentage of difference on my own and I need to know how the Boxster 986 determines it's speed. Front tire rotation speed, rear tire rotation speed or neither?

In this case he was being a jackass.
Then get a friggin stand alone GPS

People who live in glass houses shouldn't throw stones

Maybe you should ask your hero Trump for the answer.
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Old 04-18-2019, 08:50 PM   #18
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Then get a friggin stand alone GPS

People who live in glass houses shouldn't throw stones

Maybe you should ask your hero Trump for the answer.
Why? We have speedometers built into our vehicles for a reason, no need to spend money on stuff I don't need and would distract my driving.

I don't live in a glass house. Do you?

Bringing politics into this thread, damn now your digging hard with that crap.
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Old 04-19-2019, 07:16 AM   #19
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WOW! This thing devolved rapidly.

My observation is that 99% of the people who contribute to this forum are not 'jackasses' and Qmulus is certainly far, far from being a jackass. The automatic assumption that an answer that doesn't satisfy a question makes the guy a 'jackass' is ridiculous.

I think everyone needs to take a deep breath and learn to get along. There is far to much friction in the world already and it shouldn't be brought into this forum.

That's it for my soap box speech.

Happy Boxstering everyone!
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Old 04-19-2019, 09:04 AM   #20
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Reading prior posts by those involved in this leg wetting contest might bring to light who the jackass is. Don't have a dog in this fight, but that may be a reasonable solution for those that really need to be right.
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