Speedometer error

I have read the previous posts on the apparent speedometer error in the P-cars. Mine reads 53 when I'm doing 50mph. Can this be fixed by the dealer? I have a 04 C2. Thanks.... Dave

I'd be interested in this too, as I also have this apparent issue... Big6, do you have any performance modifications on your car or is it stock? I only ask because I was thinking this might be the reason, but I could be totally wrong..

do you per chance have 19" wheels on your car? that would cause it...

Hi Guys, My car is a stock (so far) 04 C2 with 18" stock wheels. I thought I read a post some where that a software change could correct this. I'm not too concerned about the speedometer error but more so about the odometer error. If I drive 50 miles and the odometer shows 53 miles, that will shorten the warrantee by a significant amount..... Dave

There've been many previous posts on both the odometer & speedometer error..Was told by Stevinson Porsche in Denver that Porsche feels that any error within 10% is acceptable and will not make adjustments.

Most manufacturers have a built in error of several percent...as I remember reading BMWs seem to be the worst. The only thing that is not legal [at least here in the US] is for the speedometer to be reading below your actual speed, so I think most manufacturers build in a higher number....it should work OK for you--3MPH at 50 isn't that bad [plus when you show 80MPH you'll be going slower in actuality]...I'd take it for what it's worth and not worry about it--it could save you from a few tickets [or help you pay less if you do get one] my $.02

The only problem is that your odometer will reach your warranty limit sooner than it should.

I have done numerious tests of my car, including several measured miles, and the odometer is exact at all speeds. I regularly travel to a place 123 miles away and have done so it 5 different cars and they all report the same mileage withing 1/2 mile.

I don't think mine is accurate. Hey, how do you like the GT3 Wheels? Did they fit okay? I was told that the fronts would rub at full lock.

Bought the car with the wheels and have had no problems.

Stick a GPS on the dash and get a reading from that if you want to find out how much error you have. Then you can decide whether this is a warrenty issue...

The speedo has a 2 MPH offset built in as near as I can tell. Reads 2 MPH fast at all speeds. Might save you tickets?

I placed a high end Garmin GPS on my dash and at almost every speed there was a 3 to 4 mph difference in reading. My 99 996 C2 with 17's read 3 -4 miles slower than the GPS was registering. Porsche considers this well within their margin of acceptable tolerance and that the speedo is an "approximation" of speed only IOW, Porsche considers this normal and it isn't an issue they are considering the issuance of a technical bulletin. I also heard, though, I don't know how accurate this might be, but that they purposely gang adjust their units to read slow so that there is less liklihood of speeding tickets---again, I don't know if this is even remotely true. Jim

The odometer is correct it is the speedometer that has a 2-3 mph offset. Notice I said offset not error. This is built in on purpose. For what purpose I don't know. The Germans have there reasons but so far have not told us. My Toyota is right on. Again I believe this is also on purpose.

Thanks all....I'm hopeing for a GPS unit for Christmas so I'll check out the odometer readings.. Dave

I just compared the displayed digital speed against a gps and from 30-85 mph the gps indicated a speed about 3.7 mph less. I used an old gps that did not measure distance traveled. Anyone compare milage and gps milage to see if odometer is effected by speedo error? Thanks.

Never have. My GPS showed 119 when the digi speedo showed 121

Was that you on the Beaverton Hillsdale Hiway?

i have one of these 'black boxes' in my 951 to adjust for my larger-than-stock wheels, but it could also be used to fine-tune your speedo if wanted. as measured by GPS, mine is dead-on now. http://www.atrol.com/era.htm from their site: Any change in a vehicle's tire size or rear axle ratio will cause the speedometer (and odometer) reading to be inaccurate. The Electronic Ratio Adapter (ERA) is designed to correct this problem. It receives the current speedometer signal from the vehicle speed sensor and sends the correct speed signal to the speedometer. The ERA installation is a snap with only four wires (input, output, hot, ground). Set the dip switches following the Abbott Calibration Table and your ready to go. The ERA has one of the widest pulse ranges available.

reading 4+ mph faster than the radar warning set ups along the highway.

I have compared mine (99 with 18 inch wheels) to the GPS and the speedo was 4 mph fast at all speeds above 15 mph. I have compared the odometer to the speedo check signs on the highway and it was dead on. It doesn't bother me at all. Kris

If you are trying to get the digital speedometer to match the analog (needle) speedometer, it can be adjusted by pulling the needle and re-adjusting it. I had to do this when I changed the gauge faces from black to white. A little trial and error ended up with about perfect sychronicity!

Hey guys, Is this manual approach the only way to get dil to match 'real' gps speeed? Thanks Matt

Quotes source post: 1785634

for what its worth. I have a radar detector which shows speed. A Garman which shows speed and when i am driving, nothing matches up with my car speed reading. They are all 5mph out at 50mph. I have 18 inch wheels on mine. I havent even noticed if the digital speed matches the the other speedo on the car. I don't give a Sh** what they say as long as they are working. If a cop asks me what speed I was doing after he pulls me over. I will say no, I was hoping you could tell me haha.

This topic was big in the late '90s with new owners. The dealer change the speedo to read closer to the actual miles per hour. Had it done in my wife's Boxster in about '01 or '02. The dealer did it at no charge on my last service before the warranty expired. Wow, time flies. The Boxster has not been to the dealer or any shop since. It's been a very trouble free car and still looks like new.

No kidding time flies Nick.....this tread is 8 years old.

My speedo is seemingly about 4 mph off as well. I'm glad to read that the odometer is not affected by this.

Yeah old ... but its good plus I believe in keeping thread together - ask himself. I should explain why I ask cos it's not a cop concern. I ask not because of odo either (not affected as you'll understand in a sec) but because I can pull the kph off the car's ecu via the odb2 cable and that kph is .... wait for it ... almost exactly the same as the gps! So the car's ecu knows the 'real' speed but for various road-use reasons discussed above and on a couple other threads the kph or mph displayed is different. So why care? Well when tracking the car, one way to know if you've improved how you take a corner is to see your exit speed, or likewise your top speed on the straight. It gives you something beside just how it feels to work with. Now using data from my gps/data logger I see one number - the ecu's - during analysis and another number when driving - the dial's. And as you know the difference of a second on the track can be determined by only a 2 kph difference in 3 or 4 corners. So while I'm driving I have to mentally be adjusting the speeds to match the analysis - not ideal. My OPC says it can't be done using PIWIS etc... But I always ask RL first. We are not the Borg but definitely THE collective (c). What say ye motley crowd?

Just to update things on the thread, the DOT mandates that we get an optimistic speed display. However, the ODOMETER will read precisely the measurement of a mile. There is another speed that is never displayed but resident within the cars' logic circuits--its actual speed. That one is necessary to do things like control raising the spoiler at 75 mph. It's supposed to come up--but if you notice when you're doing an indicated 76 mph the spoiler remains down. Because of the built in fudge factor, it will take a little more indicated to finally deploy your spoiler.

Thanks Dan - that's a reall neat way of confirming what I said re odb2 ecu data. But my question is CAN it be modded back to accurate (not MAY it)?

I'm about 4 miles high at 60 mph too.....

Interesting thread. I noticed this too, though what I noticed first, before I had a GPS data logger (AIM Solo) I noticed the odo being 5% off. GPS confirmed that with speed....5% off . This is not on the track, this is on the street. One day on a long trip I started to notice that it would take 9.5 miles on the mile markers to go 10 miles on the odo. It's been consistent since then no matter what road. Stock wheels, stock tires, on a GT3. When I bought the AIM, I played around with it on the street and found the same, including the speed differential. This unfortunately means our mileage is going to be way off with all the miles we drive it (23,000 odo miles last year)

Naro that's a new problem in a sense as the ecu in the car should control the odo and the car's ecu is much closer to the aim gps derived speed. Maybe I'm wrong and the odo isn't linked but I've had some experience with the instrument cluster - funny story ... not, but my OPC was trying to fix a cluster problem I had and all of a sudden my 18,465km car was reading 42,400!! and the ecu thought the car was a c4s!! Seems a PIWIS bug/inexperienced tech wrote data to the cluster and somehow tranferred the readings of a prior car to it - very odd and lots of blushing..mumbling etc... and to tell the truth after missing the car for 2 weeks I just moved on. They had it hacked and removed by an russian (non-P) and fixed ... but long story short the odo was getting it's data from ECU not based on speedo . OT: to continue the saga a little, I now has a remaining problem in that my CDR23 went into protect mode and I can't get it out. Have tried all the stuff mentioned on RL. But it seems to be an after effect of the russian. The cluster speaks to the radio to transmit station choice to cluster readout I think and that's most likely got a fault. OPC can't fix with any of their PIWIS. Any experience in this regard would be most appreciated as the radio turns itself on and is programmed to a hindu music channel by some unfortunate coincidence. Might be just a sign that its time for the old radio delete

The problem is true of all three BMW cars I own and all P cars I driven as well.

Quotes source post: 9446780

This is not a problem, I remember the notice about this in one of the official porsche manuals. The difference changes too depending on your speed, after 100km/h it 6km above and its 4 km for anything less than 100 if I remember correctly. If you want to see the "real" speed you can change your AvAC unit to display that as one of the output options.

Quotes source post: 9446331

Don't know why I feel argumentative today. Maybe that time of month... And I'm only picking (and only a little) on HiWind because he is a fellow track guy. So no real harm intended. So my comment is: I don't think it matters 2c what the speedometer reads on the street or on the track. Once you have calculated what your specific variance is on your car, then that number is in your head forever and you compenstate accordingly anyway. It's no different than always setting your watch 5 minutes ahead. You KNOW it is five minutes ahead and you compensate. Same applies to speed on the track. If you make a change to the car and want to know how much that change improved things, it matters not what your speedo says in "real" or "indicated". If you are using the speedo as the measuring tool and the speedo reads higher than before at the exit from the turn, then the change improved things, etc. The "real" speed vs the indicated speed is irrelevant. Otherwise REAL race cars would have speedo's... And more speed on exit means more speed to scrub off when you reach the next braking point. So getting a faster time means improving all the factors (including the size brass you use for your ***** when you enter that next turn 5mph faster than before. LOL!)

Hey TT - always glad to have some robust debate! Especially w a grizzly old tarmac warrior Actually I think that's they key. Your experience allows you to 1. Remember lots of numbers 2. Feel the force (Luke) and 3. Live by the seat of your ... ***** - kevlar or carbon fibre as they maybe . I'm a enthusiastic beginner in my first season and so the more science I can bring to it, and the better my learning aids, the quicker I'll find the limits I need to know about. My coach is a busy guy - does stunt driving and journo stuff too - so I record his laps and then try replicate and I record them w the GPS ie ecu speedo ... So the last thing I need/want is to be transforming his 145kph from ecu to 152 on speedo as I'm going into 3 after breaking at the 2nd tree, increasing the turning angle just after the bump, keep the revs at 5.2 to 5.5 and aiming for the end of the curb .... If you know what I mean. I hope in time I can train my inner Jedi to just feel, but until I can I must use the numbers. Two sides of the same coin I recon - the old and the noob You mention setting your watch 5min ahead. I feel like I have 2 watches!

Quotes source post: 9447311

Hey Mike - what's a AvAC?

AvAC are those little windows on your heat/AC control panel that tell you what temperature you have set the system. There is a great thread about how to change the readout to read any of several dozen different things. Search for this thread. You may find it useful, Young Skywalker. And please enjoy your track time and gleen all you can from your mentor. My only further comment on that area is that it seems easy to seek maximum technology in the beginning to solve lap time issues, when what is really needed is seat time and honing of the skills. Like any athletic endeavor, high performance driving is a SKILL. It takes time, and it takes frequent reinforcement to aquire and maintain those skills. Welcome to the Dark Side of the Force. LOL!!!!!!! Oh, and an old Chinese proverb I love to trot out now and then: A man with one watch always knows what time it is. A man with two watches is never certain. LOL!!!

I seem to have read somewhere that the speedo variation from actual speed is due to EC regulations that forbid sppedo error on the down side, i.e less than actual. And so EC cars err on the side of caution, usually by factor of 3> than actual. Every single BMW that I owned prior to the C4s exhibited this difference somewhere above 50mph. In the end who cares? Not really material.

Thanks DarthTub! Lol! I'm not arguing on those points - the more track time the better for sure. and thanks for that last quote - its exactly right! Help me get rid of the second watch (speedo!) why dontcha! actaully that AvAC idea could be perfect except it's not in the right place, but promising none the less. But it's the first answer so far .... Ding.ding.ding Thanks! oh wait, it came for that other guy