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THIS DISCUSSION / SOURCE-DERIVED OVERVIEW
Discussion summary
YOUR RESEARCHOBD speed does not match
Leading reported evidence
Replace oem 18" wheels
Performed · outcome unconfirmed
No successful repair reported for this finding.
0
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0
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1
Discussions
Read evidence and context
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Performed · outcome unconfirmed
This difference did not change when I replaced the OEM 18" wheels with OEM 17" (yes, I went smaller!), and my faithful Garmen has confirmed the discreptancy.
Leading evidence: 'Replace oem 18" wheels' (no explicit attributable resolution; this is a reported action or suggestion). Across 1 matching discussions, The most frequently discussed action is 'Check yet' (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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Suggestion
Don't know about the 996 as I haven't done a GPS check yet.
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Suggestion
If the oil change uses the e-oil level reading to measure the amount of oil in the engine -- and many who change the oil use this -- then the level could be off as the wrong thing is being used to measure the amount of oil in the engine.
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Suggestion
Also, based on limited testing the cushion doesn't appear to grow as speed increases which one would think it would to provide even more cushion.
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Suggestion
The obd 2 computer must send both and the software chooses which one to use???'
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Performed · outcome unconfirmed
This difference did not change when I replaced the OEM 18" wheels with OEM 17" (yes, I went smaller!), and my faithful Garmen has confirmed the discreptancy.
0 original-poster resolution reports. Counts can overlap; they are not unique people or a success rate.
0 discussions have direct query context; 0 report a fix in that context. Other evidence may inherit the discussion’s subject.
Suggestion
If the oil change uses the e-oil level reading to measure the amount of oil in the engine -- and many who change the oil use this -- then the level could be off as the wrong thing is being used to measure the amount of oil in the engine.
0 original-poster resolution reports. Counts can overlap; they are not unique people or a success rate.
0 discussions have direct query context; 0 report a fix in that context. Other evidence may inherit the discussion’s subject.
Suggestion
If the oil change uses the e-oil level reading to measure the amount of oil in the engine -- and many who change the oil use this -- then the level could be off as the wrong thing is being used to measure the amount of oil in the engine.
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Suggestion
(Mode 1, Pid 0x0D or 13 decimal.) Based on my data monitoring in my cars the speed obtained via the OBD2 port agrees with the GPS speed and also with (FWIW) the roadside speed signs that use radar to monitor vehicle speeds.
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20 eligible posts · 0 unverified posts · 1 discussions match the subject in their title or opening post.
Rennlist: 1 discussions
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Generation: 20261027T053112Z · outcome-qualification-6. Deterministic extraction; no AI model inference. How we use sources
My first post here as a new 04 C2 owner. Beautiful black coupe with tan interior. I have a couple of questions. First, how reliable is the electronic oil level? Mine showed about half full but the dip stick was at the bottom of the range; both measured at the same time at operating temperature. Second, using an ELM 327 OBD-II with EnginelLnk software the mph does not match, about 5 mph difference.
Briefly, at oil change time the e-oil level system's function is verified by following a specific oil drain procedure and then afterwards adding a specific amount of oil and then confirming the read out agrees. In the case of for instance my Turbo 7.8l of oil is added then the level checked and it should be at the max line, not above, not below.
As for the dipstick vs. the read out the dipstick may not have the range the e-oil level system has. The electronic oil level sensor has a good amount of bare wire used to measure the oil level (I took one apart to see how it worked) while the dipstick tell tale is just an inch long if that much.
If the oil change uses the e-oil level reading to measure the amount of oil in the engine -- and many who change the oil use this -- then the level could be off as the wrong thing is being used to measure the amount of oil in the engine.
As for the speed discrepancy that's a common issue, though minor. My Boxster's speedo is optimistic by 5mph while my Turbo is optimistic by just 2mph. Early in my ownership of my Boxster, based on a test drive of 200 miles the car's odometer (trip odometer) disagreed with a GPS distance reading by just 0.2 miles. Assuming the car's odometer was the optimistic instrument, given a 50,000 mile warranty that's 250 200 mile trips and 250 times 0.2 miles is 50 miles so the odometer reading would be off by 50 miles at 50K miles.
Welcome to the forum, I believe I have the same car, black w/full natural leather, Technic, PSE, Lt. Carreras wheels,'04 C2 . purchased from Paramount motors in N. Carolina. Got any pics?
I think the speed difference is described in the manual. If not it's in the technical manual. Bellow 40 milles its 3 miles above 40 miles its 5 miles or something like that (I deal with km). You can use the AC unit to display the correct speed and that is what the odometer goes off. Its a safety feature.
Some ODB2 show the right speed too. My laptop based one is off like yours but my Car Stereo based one is right on. The obd 2 computer must send both and the software chooses which one to use???'
There is only one OBD2 speed parameter. (Mode 1, Pid 0x0D or 13 decimal.) Based on my data monitoring in my cars the speed obtained via the OBD2 port agrees with the GPS speed and also with (FWIW) the roadside speed signs that use radar to monitor vehicle speeds.
It appears for some reason the instrument cluster controller modifies the speed and over reports it, at least in the case of my Boxster and my Turbo.
Some claim it is a safety feature, but why for instance with my Boxster does Porsche believe I need a 5mph safety cushion while with my Turbo I only need a 2mph safety cushion? (IIRC my 08 Cayman S's odometer speed agreed with its OBD2 and GPS speed.)
Also, based on limited testing the cushion doesn't appear to grow as speed increases which one would think it would to provide even more cushion.
Others claim the difference arises from the different wheel/tire sizes a model can come with so no matter which sanctioned wheel/tire combination the car has the speedo never under reports true speed. (Some claim that some areas of the world -- in Europe for instance -- have strict laws that punish a car maker for underreporting speed.)
To accommodate wheel/tire different combinations, this to me makes some sense.
My 02 Boxster could have left the factory with 16" wheels/tires but did not.
Apparently, it left the factory with 17" wheels/tires and IIRC from back then had I ordered the car I could have optioned it with 18" wheels/tires.
Standard wheel/tire size on my Turbo is 18". I do not know what other sizes were available from the factory.
Regardless, I am not sure why the differences between true/actual speed vs. reported speed and the varying differences between models between true/actual speed vs. reported speed.
All I know is this difference in displayed speed vs. actual/true speed is a common issue with new owners who happen to spot this.
I would not and did not let it interfere with the enjoyment of my new car.
Once I satisfied myself with my new Boxster the odometer reading was not materially affected I just took (and continue to take) the difference in stride, allow for it when driving.
Instinctively, I drive 5mph over the limit based on what the digital speed reading is on the dash knowing the car is actually moving 5mph slower.
That is with 30mph indicated the actual speed is 25mph, and the same 5mph difference exists at 30mph, 35mph, 40mph, 45mph, 55mph, 65mph, 70mph, 75mph (outside of CA) and even at 80mph on stretches of Texas freeways that have 80mph speed limits. (And I can also report that at even higher speeds up to <static> that 5mph error still exists.)
I trust the speedo's optimistic reporting enough to once in a while at 70mph indicated but 65mph actual to pass slower moving police or highway patrol cars on the freeway knowing with confidence I am not running even 1mph over the posted limit.
In my 986S, the OBDII speed seems pretty much spot-on with the GPS speed, while the dash registers about 4mph high at highway speeds. This is apparently normal for German cars.
If you get a ticket for speeding and you could prove that your completely stock automobile's speedometer was reading lower then your actual speed, causing you to get that ticket, who would you sue? I'd sue the car manufacturer for much $$$. If the speedometer reads higher then actual speed you pay the ticket and shut up.
Lower reading = safety buffer for auto manufacturers.
However if the odometer is off someone will start a class action suit, NHTSA may get involved the manufacturer will be out $$$$$$$, and have an expen$ive recall, and most likely will have to extend the warranty.
My 996's speedometer is optimistic by 5 mph between 40-45 mph and 75-80 mph and consistent across this range. This difference did not change when I replaced the OEM 18" wheels with OEM 17" (yes, I went smaller!), and my faithful Garmen has confirmed the discreptancy.
On the other hand, my spouse's '09 Benz E350's speedometer is right on the button across a wide range of speeds...again, according to my Garmen.
Its a Carrera thing, I guess.
relinuca
FWIW, German cars are the worst when it comes to speedo's being off. I remember seeing a comparison online once, and BMW, VW, Porsche, Audi were all 5-6 MPH high. The American cars were 2-3 MPH high and Japanese were the most accurate staying at 0-1 MPH off actual speed limit.
I drove lots of different cars in Germany, typically they would be european cars, and there was ALWAYS a significant difference between GPS speed and speedo. I think it's partially to give drivers an increased sense of power!
I've never driven a Japanese car with a speedo which was anywhere near accurate.
All of my cayennes have been 3 mph over at 70 mph and about 6 mph over at 150 mph. Don't know about the 996 as I haven't done a GPS check yet.
My wife's '04 Toyota 4Runner is spot-on accurate to within less than 1mph per GPS and mile markers/stopwatch.
Whereas I find myself doing mental arithmetic all time time with my (horribly inaccurate) Porsche speedometer. Wonder if I get pulled over if I can plead improper equipment, even if it registers high?
