tgt40

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

NaroEscape

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)

HiWind

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

dntlvet

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

mikeswift

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.

Thundertub

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!)

HiWind

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!

HiWind

Hey Mike - what's a AvAC?

Thundertub

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!!!

ogun228

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.

HiWind

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