inaccurate speedo

[u63af]

Ever since I have had my 944 turbo, (4 months) the speedo seems to be inaccurate. IT says I am doing 100 when I am doing about 80 mph. I could live with this as my sat nav has a speedo but it is putting on more miles then I am actually doing. Is there a common fault with these such as the speedo sensor or can it be a gauge problem?
Cheers
Sandy

 

[TTM]

Above 60 mph both my 944s show an average difference of 12mph between what the speedo and the GPS read. They are both on original wheels and tyre sizes.

 

[James_G]

Mine is 10% out, so and indicated 33 is 30mph and an idicated 80mph is 72mph. And it's on totally standard wheels and tyres.

Your difference sounds high though - I would second the wheels issue.

 

[pauljmcnulty]

Mine's about 10% over. Not a problem, as I also have a GPS that I use for accurate speed. Everything's standard, no fancy wheels and probably only the second set of Pirelli P6000s. No appreciable wear on them yet. [&:]

You do raise an interesting point though. Should I revise my recorded 135K miles down by 10%? [8|]

 

[idwoodvale]

Both my 944 and Volvo are out by about the same. 80 on the speedo reads about 73/4 on the Tom Tom IIRC. Noteably though, when driving through road works and keeping to exactly 50 on the speedo with cruise control on I seem to be doing exactly the same speed as all the other cars - I have assumed that either there is a deliberately inbuilt margin of error which is fairly standard across all cars, or that the Tom Tom is inaccurate (or both).

 

[u63af]

I have replaced the wheels with 17 cups but the rolling diameter is the same as the standard d90s. The speedo would only over read if I put wheels on with a smaller rolling diameter. It did it when I had the d90s on aswell. I assume that the sensors on the 944 turbos is an electronic one? My old 924 turbo was a cable through the front hub. It is just annoying me as it is putting on so many miles that I am not actually doing.

 

[sawood12]

Well if you really believe your GPS is absolutely accurate (I think people over estimate the accuracy of GPS) then the biggest issue here is that your speedo is over reading. Normally speedo's are designed to under read so you are not doing a higher speed than indicated.

 

[ikillcopiers]

I have to agree with sawood.

GPS's get their speed by an average, over a few seconds.
Also the tolerance is quite high - car GPS systems are usually
accurate to around 10 meters, the logic being at 10 meters you
can see where you're meant to be going.

If you really want an accurate benchmark, you need to get something
like a Performance Boxhttp://www.performancebox.co.uk.

These things are designed for accurate speed testing,
most of the car magazines use these, and similar things are used
in motorsports for datalogging.

 

[sawood12]

Also with GPS there are two systems, the military system and the civilian system but both are owned by the US military. The military system is very accurate as it is guiding smart bombs, the civilian systems is nowhere near as accurate - and the US don't want it's enemies to have the same accuracy of GPS as they do or their own system will be used against them. At the beginning of Gulf War Part I due to a shortage of supply of military GPS handsets the US military temporarily increased the accuracy of the civilian signal so civilian GPS units could be used. Now i'm not sure if they have switched this more accurate signal back - they certainly were going to for National Security reasons. So your Tom Tom or whatever other GPS unit you have will be sampling it's speed of pretty inaccurate positions - and the accuracy is in 3-dimentions so that is a power of 3 level of inaccuracy and not a straight 3-times level of inaccuracy.

 

[idwoodvale]

ORIGINAL: sawood12

Well if you really believe your GPS is absolutely accurate (I think people over estimate the accuracy of GPS) then the biggest issue here is that your speedo is over reading. Normally speedo's are designed to under read so you are not doing a higher speed than indicated.

Would it not work the opposite way? If your speedo says 90 and you're actually travelling at 70 then it is over reading and you're safe. If it under read then at an indicated 70 you could be doing 90.

 

[uknick]

ORIGINAL: idwoodvale

ORIGINAL: sawood12

Well if you really believe your GPS is absolutely accurate (I think people over estimate the accuracy of GPS) then the biggest issue here is that your speedo is over reading. Normally speedo's are designed to under read so you are not doing a higher speed than indicated.

Would it not work the opposite way? If your speedo says 90 and you're actually travelling at 70 then it is over reading and you're safe. If it under read then at an indicated 70 you could be doing 90.

I agree, do speedos not read 10% over, i.e. 30 instead of 27. The automatic 30 MPH signs around Ascot flash at me when I am doing 33 mph. But, on the other hand the signs in Ashford, Surrey, flash when I am indicating 29 mph on the digital speedo. Now I am not sure what is happening [&:]

 

[mikead]

Just found this if it helps?

"The Road Vehicles (Construction and Use) Regulations 1986, as amended, allows the use of speedometers that meet the requirements of EC Community Directive 75/443(97/39) or ECE Regulations 39. Both the EC Directive and the ECE Regulation lay down accuracy requirements to be applied at the time of vehicle approval for speedometers. These requirements are that the indicated speed must not be more than (+)10 per cent of the true speed plus 4km/h (2.5 mph)." "In production, however, a slightly different tolerance of (+)5 per cent plus 10 km/h (6 mph) is applied" "The requirements are also that the indicated speed must never be less than the true speed. A vehicle meeting these requirements would not be able to travel at a greater speed than that shown on the speedometer and a driver could not, therefore, inadvertently exceed speed restrictions. Her Majesty's Government have no plans to introduce instrument tests"

 

[944 man]

ORIGINAL: sawood12

Also with GPS there are two systems, the military system and the civilian system but both are owned by the US military. The military system is very accurate as it is guiding smart bombs, the civilian systems is nowhere near as accurate - and the US don't want it's enemies to have the same accuracy of GPS as they do or their own system will be used against them. At the beginning of Gulf War Part I due to a shortage of supply of military GPS handsets the US military temporarily increased the accuracy of the civilian signal so civilian GPS units could be used. Now i'm not sure if they have switched this more accurate signal back - they certainly were going to for National Security reasons. So your Tom Tom or whatever other GPS unit you have will be sampling it's speed of pretty inaccurate positions - and the accuracy is in 3-dimentions so that is a power of 3 level of inaccuracy and not a straight 3-times level of inaccuracy.

'Selective availability' was turned off by Bill Clinton almost a decade ago! Whilst a GPS receiver wont be as accurate as some people assume: a steady readout will be accurate to within a mile an hour or so.

My series one car is accurate to one mile per hour up to 100mph: due Im sure, to the 235/45/17s increased rolling radius...

 

[u63af]

I think I will try changing the speedo sensor.

 

[sawood12]

ORIGINAL: idwoodvale

ORIGINAL: sawood12

Well if you really believe your GPS is absolutely accurate (I think people over estimate the accuracy of GPS) then the biggest issue here is that your speedo is over reading. Normally speedo's are designed to under read so you are not doing a higher speed than indicated.

Would it not work the opposite way? If your speedo says 90 and you're actually travelling at 70 then it is over reading and you're safe. If it under read then at an indicated 70 you could be doing 90.

The original poster said (according to GPS) he was doing 70mph with a speedo indicated 100mph so I meant that it is over reading relative to true speed.

In any case i'm not sure that the signal that is driving the speedo is driving the odometer. The mechanical drive for the speedo is converted into an electrical signal whereas the oddometer is wound mechanically, so if there is an electrcial gremlin meaning you are not getting an accurate speed reading your oddometer should still be working OK. Of course it is still susceptable to differences in the rolling circumference.

 

[dirtydirtyharry]

ORIGINAL: mikead

Just found this if it helps?

"The Road Vehicles (Construction and Use) Regulations 1986, as amended, allows the use of speedometers that meet the requirements of EC Community Directive 75/443(97/39) or ECE Regulations 39. Both the EC Directive and the ECE Regulation lay down accuracy requirements to be applied at the time of vehicle approval for speedometers. These requirements are that the indicated speed must not be more than (+)10 per cent of the true speed plus 4km/h (2.5 mph)." "In production, however, a slightly different tolerance of (+)5 per cent plus 10 km/h (6 mph) is applied" "The requirements are also that the indicated speed must never be less than the true speed. A vehicle meeting these requirements would not be able to travel at a greater speed than that shown on the speedometer and a driver could not, therefore, inadvertently exceed speed restrictions. Her Majesty's Government have no plans to introduce instrument tests"

I went on an advance driving course with a police instructor and was told about this. Apparently all modern cars have a speedo which over-reads by 10%. A lot of people dont know about this including many police officers!

I know its not best practice but it means that you can actually overtake a cop car on the motorway where he thinks he is doing 70mph but is in actual fact only doing 63mph! [].

I used to rely on my sat nav unit to give me my true speed but since i changed cars and dont have it anymore i now add 10% to the speedo. Therefore im confident driving with my speedo showing 77mph.

If you think about it car manufacturers are bound to put artificially high speedo's in cars - this is a compensation society we live in and can you imagine the court cases if manufacturers were to use 'precise' speedos which could 'drift' slightly with time causing the driver to break the speed limits!!

 

[944 man]

Thats not really accurate; cars do not have a speedometer that over reads by exactly 10%. If it were so then theyd have one that was absolutely accurate and youd certainly know about it. Cars must have a speed indicating device that does not under read under any circumstances; and does not over read by more than ten percent. Both tolerances must be maintained across full tyre wear and the leaway allows for both tyre wear and manufacturing tolerance.

The odometer in a series two car is driven from the speedometer drive in a similar fashion to the odometer in the series one cars, so if the speedo is reading forty-odd percent high, then youll be registering 100 miles for every 70 miles travelled.

Simon

 

[sawood12]

Well i've not actually been round the back of my speedo to check but the chap I bought my EBC off who had it in his turbo hooked it upto the electric input of the speedo as you can programme different boost levels in different gears and the EBC determines which gear you're in from marrying up and learning your RPM vs Speed relationship. I've had this verified by another independant source. So not sure how the speedo is driven but there is definately an electric speed related signal going into the back of it.

 

[u63af]

I saw a speedo sensor on ebay recently for a 944 turbo and it did not look like it had a cable coming from it like the earlier cars. That is why I assumed that it was electrical but I could not see the whole thing.

 

[944 man]

It doesnt have a cable: its an electrical sender that fits into the gearbox. If youre confused by my post above then I apologise for my lack of clarity. The electrical signal is converted in the binnacle and the odometer is driven from the analogue side, after conversion.

I think that talk of boost controllersis misleading. Extrapolating the selected gear from the speedometer and the tachometer readings is relatively straightforward, but the car isnt doing anything more complicated than converting a pulse signal into an analogue readout.

Simon