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Old 03-16-2021, 11:07 AM   #1
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MPH Issue Revisited

Hello to my fellow 986er's. I accidentally read about the Speedometer showing too fast in the Boxster ... mine is the 1997 version. At first glance it did not seem to be a big deal until I started digging in ... with 5/32nd tread left on my rear tires (255/40ZR17), my speedometer at 70mph GPS, shows about 74-75mph. The only times in the past that I have done a basic check is when I pass one of the convenient watch your speed displays on the side of the road. Almost every time in different vehicles, my speedometer seemed to match what was displayed. Not so with my project 986. So I read a little more on different forums and was a little relieved to learn that even though the speedometer was off, the odometer was apparently true.

Being the curious, detailed person I can be sometimes, I tested this theory by driving 80 miles on the interstate at a steady 70mph using my handheld GPS for accurate MPH. After 80 miles my odometer showed 2 miles over actual travel distance ... so roughly 1 mile too much every 40 miles. With 86000 miles on the project when I purchased it ... that would add up to about 2,150 miles too many on the vehicle over its life.

So lastly I computed in tire wear over time on an average mph loss due to wear and came up with about 663 miles too many if the speedometer was dead on at 70mph at the same 86,000 miles as delivered ... roughly 69.5mph average due to wear.

Rhetorically speaking, how did German engineers get away with such lack of detail? Did they eventually stop programming this error into the Speedometer in successive generations?

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Old 03-16-2021, 11:40 AM   #2
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I just read somewhere recently, (and don't remember the source,) most speedometers are supposed to read 2-3 mph fast.
I wouldn't sweat it.
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Old 03-16-2021, 12:15 PM   #3
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All factory speedo's were set 3-5 MPH too fast for two reasons: (1) So you couldn't sue the OEM when you got a speeding ticket; and (2) To run out the factory warranty faster..............
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Old 03-16-2021, 12:16 PM   #4
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I have read the same thing. My gps based speedometer consistently reads 2 mph slower than the speedometer in my 00 986. Not sure the reason why (oversight, an attempt at keeping the speeding tickets away, or just making the driver feel fast).

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Old 03-16-2021, 07:25 PM   #5
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Hey all. Not to be too big of an math nut on you here, but 1 mile every 40 miles isn't really unreasonable. For my 265/35R18 rear tire, with ~8/32 to 10/32 of tread depth throughout the life of the tire, I did some of the math and just the theoretical change in circumference of the tire over it's life would result in an additional 0.81 miles per 40 miles on a "worn out" tire (2/32" of tread depth remaining) compared to a new tire. I think adding in the fact that the Boxster came with different diameter wheels wearing different section width tires, that'd be within the realm of engineering accuracy? What really baffles me is that my 01S seems to always indicate 4 miles per hour faster on the speedo than whatever speed I am doing. How that magic happens, I'm not sure, because the speed differential should be proportional to vehicle speed if it's based off of a sensor and an algorithm.

Anyway that's just my two cents.
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Old 03-17-2021, 07:40 AM   #6
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Pert-near Engineering

I like the run out the warranty sooner thought JFP. Sounds like a 5 O'clock Friday afternoon fix to me. ehh, that's close enough ... whatever their reason for pert-near engineering, I remember my electronics teacher would pull out everything on my bread-board if it wasn't meticulously neat.

Now I am going to have to test all of my vehicles for actual speed and odometer readings for comparison ... that's my $500 dollars worth.
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Old 03-17-2021, 08:11 AM   #7
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So everyone uses GPS to verify speedometer readings.
Any GPS I have ever used has an error factor when showing exact location.
They can get you within say 15 feet of a specific spot or 20 feet or what ever the error factor is at that particular time and location.
So are they spot on accurate when it comes to speed??????
If I wanted to verify my speedo I would use a measured distance over time.
I would think that would be the most accurate.
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Old 03-17-2021, 12:45 PM   #8
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Yep, and furthermore this isn't advanced technology controlling our odometers. It basically counts wheel rotations and multiplies that by the circumference of the tire. The circumference changes so it will never be precise.

Last edited by porschefan76; 03-17-2021 at 12:48 PM. Reason: Clarity
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Old 03-18-2021, 04:53 PM   #9
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I agree that the Porsche speedometer reads higher than actual speed, and that tire wear will affect both speed and odometer readings. But I believe the odometers are more accurate than the speedometers. There have been lawsuits against auto manufacturers in the past for odometers reading higher than actual to make warranties end sooner. And the speedometer readings seem to be a constant 2-3 mph off regardless of actual vehicle speed. If the speedometer being high was directly related to the odometer reading, then the variance between indicated speed and actual speed would be greater the higher the speed of the vehicle. Just my $0.02.
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Old 03-18-2021, 05:39 PM   #10
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Quote:
Originally Posted by PaulE View Post
I agree that the Porsche speedometer reads higher than actual speed, and that tire wear will affect both speed and odometer readings. But I believe the odometers are more accurate than the speedometers. There have been lawsuits against auto manufacturers in the past for odometers reading higher than actual to make warranties end sooner. And the speedometer readings seem to be a constant 2-3 mph off regardless of actual vehicle speed. If the speedometer being high was directly related to the odometer reading, then the variance between indicated speed and actual speed would be greater the higher the speed of the vehicle. Just my $0.02.
I believe this to be correct as well. If I recall correctly, Society of Automotive Engineers specifies/requires odometers to be +/-4% of actual.
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Old 03-19-2021, 10:53 AM   #11
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As long as you have DGPS, the accuracy is centimeters ... most handheld GPS don't have that capability ... I had to modify mine with a DGPS antenna when I was first learning about them. Basically you have close enough to hurt someone using only the satellites ... adding the D, gives you coordinate correction based on a physical location on earth. This gets you close enough to vaporize your target. I was involved in marking locations at a shipping port so we could keep track of the location of each container. Took a while to get the data, but very much improved the location checkers job.
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Old 03-19-2021, 11:37 AM   #12
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Originally Posted by freserf View Post
As long as you have DGPS, the accuracy is centimeters ... most handheld GPS don't have that capability ... I had to modify mine with a DGPS antenna when I was first learning about them. Basically you have close enough to hurt someone using only the satellites ... adding the D, gives you coordinate correction based on a physical location on earth. This gets you close enough to vaporize your target. I was involved in marking locations at a shipping port so we could keep track of the location of each container. Took a while to get the data, but very much improved the location checkers job.
Hi freserf, so I'm just trying to understand, are you trying to theorize a more accurate approach to operating an odometer than what the 986 uses?

The 986 literally uses a sensor that counts rotations and multiplies that by a constant, some statistically validated constant, that attempts to account for the variable diameter of all possible tires offered on the vehicle. It is not complex at all. Most other vehicles also use this method.
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Old 03-19-2021, 12:18 PM   #13
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Quote:
Originally Posted by freserf View Post
As long as you have DGPS, the accuracy is centimeters ... most handheld GPS don't have that capability ... I had to modify mine with a DGPS antenna when I was first learning about them. Basically you have close enough to hurt someone using only the satellites ... adding the D, gives you coordinate correction based on a physical location on earth. This gets you close enough to vaporize your target. I was involved in marking locations at a shipping port so we could keep track of the location of each container. Took a while to get the data, but very much improved the location checkers job.

My Dad was able to see the Cuban's building a missile launch site in Oct.1962 using aerial photography.
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Old 03-19-2021, 05:53 PM   #14
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Quote:
Originally Posted by freserf View Post
As long as you have DGPS, the accuracy is centimeters ... most handheld GPS don't have that capability ... I had to modify mine with a DGPS antenna when I was first learning about them. Basically you have close enough to hurt someone using only the satellites ... adding the D, gives you coordinate correction based on a physical location on earth. This gets you close enough to vaporize your target. I was involved in marking locations at a shipping port so we could keep track of the location of each container. Took a while to get the data, but very much improved the location checkers job.
The point of my original post is that people are using hand held GPS to verify the accuracy of the speedometer on their cars. When most likely the GPS has a greater error factor then their speedometer. That is why I said a better way to verify the accuracy of a speedometer would be to use time over a measured distance.
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Old 03-19-2021, 09:51 PM   #15
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This was explained clearly in a service memo for BMW motorcycles back in the 1980s.

"Speedometer advance" is required because it's illegal to produce a vehicle in Germany that has a speedometer that could read less than actual speed. Considered a safety issue (Germans are meticulous). While different brands and models of tires may be marked the same size, that doesn't mean that they are all actually the same size. Of course there is also tread wear as mentioned.

To combat all possibilities, speedometer advance is required.

The bulletin noted that although the speed indicated may be high, the odometer is accurate. Again, how could it be given the possible variances noted? Basically it is what will be used for warranty mileage calculation ... obviously irrelevant today with our 20 + year old 986s.

PS: I'm pretty certain that fitting tires other than approved sizes is an inspection failure in Germany. or was.
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Old 03-20-2021, 06:24 AM   #16
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PS: I'm pretty certain that fitting tires other than approved sizes is an inspection failure in Germany. or was.
And yet, Same-size tires from different manufacturers will have size differences that would astound you. Especially differences in height and circumference.


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Old 03-20-2021, 06:24 AM   #17
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My S read about 3/4mph over with its original 17" set up compared to phone. I now have a set of 18" Cayman crab claws and it's virtually identical to the phone GPS. Not saying either is accurate but they are different.

I am pretty sure on my S600 that if I change from the staggered AMG wheel setup to a different size I can alter the setup on the car via Star. I think there is a setting in the car menus to switch to winter tyres but not 100% sure on that. I imagine that would be to compensate for the same setup on all four corners.
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Old 03-20-2021, 08:03 AM   #18
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That's interesting. What size tires do you have on the 17's and 18's? I've got a set of 17's and a set of 18's for my 2003 S and the speedometer is off about the same with either set on the car. My tires are all Michelins in the OEM sizes, 205/50-17 and 255/40-17 and 225/40-18 and 265/35-18. And even with the same nominal tire size, the actual height and width of tires will vary by manufacturer.
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Old 03-21-2021, 07:14 AM   #19
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Lots of Experiences are Good?

I like the Cuban missile input ... will probably be able to track speed more accurately once Musk finishes Starlink. Any guy who can launch his own car into orbit doesn't really care about speedometers I guess ...
Anyways, I only began to research my speedo issue because my wife was following me to lunch one day and she asked why I was driving so slow. She asked if I was trying to piss off the guy behind me because he was up in my exhaust. Well, that is when I typed in the question on DuckDuckGo and to my surprise, the speedo issue has been around since the beginning and because I am only 5 months into my first Porsche project ... I am still learning all of the good and the bads of Boxster ownership. I will probably stick to the same size 17's on mine when I shoe on some new ones. These tires are 6+ years old now with good tread, but they are showing signs of age. I will be looking at the same Michelin, Continental or Bridgestone ... not sure if I will purchase summer tires or all-season.

Also, I will be looking into a more accurate speed readout ... If I can modify the pulse signal slightly, it should give me the ability to correct the error. Maybe if I find the time.

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