Speedometer accuracy

[docdic]

Hi,
Does anyone know how accurate their computer speed reading is?
I understand that all manufacturers build in an over-estimation but is there any experience as to what extent Porsche has done this?
I also understand that GPS experts say that a GPS reading, when travelling on a straight level road, is probably the most accurate figure which most of us will get under normal circumstances.
However, my 1993 911 Carrera 2 cabriolet 3.6 L with Tiptronic transmission reads between 6 and 7 miles faster then my GPS device indicates at 70mph on the car digital readout...
...this is almost exactly the same difference as with my Land Rover Defender.
The difference, of course, is less at lower speeds (except that the GPS will read zero when the car speedo reads 6mph!)
Is this why I haven't been flashed when I have gone (a little) over the limit in the zone or is the error then compounded by a built-in lee-way in the camera readings?
Richard

 

[TJW964]

Try going faster as you pass the speed cameras.
Once they start to flash you will then know the speed they are set at.

Please then post the results here.

There was a post on this a while ago. Try looking backwards in the 964 posts and I'm sure you'll find it.
From memory we were all overstating by about 10 mph at 100 as against the GPS versions.
Seemed OK at 40 and 90 is 82.

See ya.

 

[smallspeed]

gps gets more accurate the faster you go, as the time-lag between bouncing signals becomes less of a factor, plus the accuracy (from memory civilian GPS is accurate to within about 6ft) becomes less of a factor too as a percentage of distance travelled... Your GPS reciever frequency also plays a part, but most run fairly high frequency now anyway

mechanical speedo's and non-contact systems (like wheel speed sensors and things) have some hysteresis, and get worse as they get faster, so you will always get some mis-alignment, however some are a lot better than others.. My E39 5-series is within 2mph at 100mph, whereas my mates merc SL is about 7mph out

ben

 

[Gordon Attar]

Richard
Trust your GPs unit and use it to double check your speedo. I've found it extremely helpful for checking all of my recent cars
My 964 at 90mph was actually doing 82mph
My 993 at 90mph is 85mph just 2mph of on the digital
My 944 at 90mph is 84mph

Surprisingly my Xantia is 86mph and the wifes Zafira 87mph havent checked the Volvo. You would expect the Porsches to be of a better standard.

 

[macandsue]

Hot off the press. M6 Sunday Junction 26/27 23.50 hours.
The Constable said, "You have no excuse for exceeding the speed limit, your GPS is accurate."
Result ? £60.00 3 points.[][]
The speedo was reading a good 10mph over at the time.

 

[TJW964]

ORIGINAL: macandsue

Hot off the press. M6 Sunday Junction 26/27 23.50 hours.
The Constable said, "You have no excuse for exceeding the speed limit, your GPS is accurate."
Result ? £60.00 3 points.[][]
The speedo was reading a good 10mph over at the time.

Sorry to hear that. I hate speeding tickets.

(Note to self : Stop collecting speeding tickets)

If you were in a 964, can I guess that you were approaching three figures at the time?

What ever happened to the 10% for speedo innacuracy?
Perhaps you used that up with your 10mph over?

Keep smiling. They go away after a few years!

PS - For the London lot, J26/27 is 'up North' proper!

 

[Mark Elder]

ORIGINAL: macandsue
M6 Sunday Junction 26/27 23.50 hours.

Ouch, I was there about an hour before you. I must have been lucky.
Was it an unmarked car?

 

[Fred Hindle]

And you have already done your speed awareness course Mac![]

I thought you didn't like motorways or had your GPS got you lost?

Cheers,

 

[macandsue]

Hi Fred,
I like them even less now.It was actually my Road Angel he could see from his very big MARKED Land Rover thingy.I was looking frontwards at the time ,there is a certain speed where looking aft is dangerous,I think I was well over that .There is a speed readout on Angel but its not for looking at its only a device to make it legal.We had covered M3,M4, Fosse Way, M40,M42, and finally M6,nearly home and I thought driving quite slowly really,I forgot about all those bridge cameras which are not speed cameras.They are .They watch you and report you to the nearest plod.If you aren't going quick enough they can catch you.Well that's my theory anyway and its obviously true.

 

[docdic]

Thank you all for your interesting replies. I am very sorry, MacandSue, to hear of your "cop". I shall trust my GPS rather than the speedo in future - interestingly, it will allow me to go a little faster before worrying about cameras...

Regards,

Richard

 

[Alexander_Drobik]

My one will be 8% out at around 40 and then gets more accurate, eg 40 mph=36ish, but by 80mph it = 76ish.....I would guess it sats 3% out to 130 - never looked above that.

 

[ChrisW]

My speedo over-reads by about 10%.

Enough to make it as fast as a GT3 (which overreads by around 5%).

 

[smallspeed]

I was bored during my 3hr treck down the M1 this weekend so tried this out with my Navman sat nav.. GPS was about 1 - 2mph under the OBC readout the whole way from 10 to 100mph... Change over point (from 1 to 2 mph) was around 45mph, where it started flicking back and forth between 1 and 2 mph difference until about 55 where its over reading by bang on 2mph all the way to 100...

pretty impressed with that!

ben

 

[911addict]

I'd say that my 964 turbo's speedo reads 10% over. On a recent track day I was clocked by the track camera at 167MPH, the satnav said 168 but my speedo had maxed out at 180 long before! []