Hopefully, the following might spark a bit of debate.

We all know that the speedo reading in cars is inaccurate - they are quoted as being inaccurate by up to 10%. But it seems to me that we all assume that they are ALWAYS 10% optimistic i.e. show a speed that is 10% higher than the true speed. Many times I've seen people say "I was doing an indicated 80mph, so in reality I was only doing 72mph". I use this logic to give myself a false sense of security when doing 90mph ("they don't do people for 81mph" I tell myself. But are we wrong to assume this? Are they ALWAYS 10% optimistic? Could it be that in many cases, they are 10% pessimistic so that they indicate 90 mph, but our true speed is 99mph? Should we in fact be making the assumption that they are UP TO 10% inaccurate in either direction? If this is so, we need to remember the term "UP TO" - in many cases, the speedo could be far more accurate than 10% out.


Comments please.
Have a good weekend.

LCG (Dave)

 

No... they always over-read

 

IIRC - due to the expense of building a speedo that is very accurate (like in a police traffic car) most car speedos are inaccurate. Now, by law they are not allowed to under read ever, so obviously they are either correct or over reading. I think again, that an overread of 10% is allowed. This does not mean that the car does overread. I think also that they have to be spot on at 30mph, but can't quite remember.

I know in my 98 5 door, upto 50mph, it typically over reads by only 1-2mph, as we have a number of the display speed traps that tells you how fast you were going and then either smiles or frowns at you, and with all of these, it usually says about 1-2mph lower than the speedo says.

[Edited by Scratch - 3/28/2003 1:39:34 PM]

 

Speedos NEVER under-read (from new) as that is actually an offence.

10% + 2 is the APCO guidelines, but with modern speedos (electronic not cable) 2% is normally reckoned to be right.

Interestingly, the SECS unit has a true speed reading converted from the ECU & it is less by about 3% from the indicated speedo, when you would expect they should be the same...

I also now from a speed trial for a magazine shoot that @ 149 indicated, I was doing 134 true...

 

Speedo's may overread by UP TO 10%, that doesn't mean they all do !
Most speedo's are electronic these days and a lot more accurate, typically less than 5 mph out at 70/80 mph.

Edited to say: Beaten to it by PTMW!

[Edited by lexicon - 3/28/2003 1:50:25 PM]

 

Mine is out by 1 or 2 MPH even at 30 going by the digital reading on my diagnostics unit compared to the speedo. And that still doesn't account for any errors in the speed sensor or gearing differences due to tyre size/tyre inflation/tyre temp etc.

There will always be an error but you can't account for how much other than it should always be between 0-10% over-reading. It's very risky to assume it will be 10% over though, especially if you're already aiming for the ACPO guideline max speed (i.e. 10%+2 as stated) after taking into account the speedo error.

 

One small point to add - as new speedos cannot under read but usually over read so that in most cars an indicated 31-32= real 30mph, indicated 73-74 is a true 70.

One way to find out is to follow a loory or two on the motorway of P-reg or newer. All p reg and newer lorries, as far as I know a legally limited to 56mph.

Top gear or 5th gear, cant remember did a feature on zero tolerance speeding and got an old sierra to check its speedo. At an indicated 30mph, the sierra was actually doing 32mph, ie under reading. It then went the other way and read 76mph or so at a real 70mph.

 

I have a SPA Micro processor in both my WRX Wagon and STi 6 Wagon.
The micro processor is as accurate as the input information ie. the exact tyre circumfrence. I have put a lot of effort into calibration and recalibrate for tyre tread wear during the life of the tyres.
On the WRX the speedometer over reads by 6% at 80mph.
On the STi I have changed from 4.44 diffs to 3.9 diffs and hence the speedometer under reads and I pay no attention to it relying on the SPA which is positioned between the dash board coaming and windscreen in line of sight.
At Elvington last Autumn I went through the timing trap at 158mph and the recall on the SPA was 157mph.
I have also noticed that the SPA consistently under reads by 5% the GPS derived speed information.
Hope all this is helpful.

 

Yeah c/o road angel GPS thing i know mine to overread by around 8% of which a slight part maybe due to me sticking needle back on the side of caution when doing white dial thing but as already mentioned it can read 0 to +10% but not underread!!!
Si

 

From a conversation I was having (via email) with a police driving instructor friend, for the informative nature of this post...

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The majority of speedos in (county name) Police traffic vehicles are electronic and specially fitted and calibrated and certified by a company in Nottingham..

Speed detection devices in vehicles are checked and calibrated over measured miles at specific locations. Calibration is usually unnecessary, unless a wheel change is called for ,which will change slightly the accuracy..however force policies vary from weekly,daily checks or checks only if a speed offence is detected.

Therefore, if you have different levels of tread on your tyres e.g newer tyres to one of the axles, I would think this would make a marginal difference to your speedo's accuracy, hence the +/- 2% tolerance. The majority of people tend to replace only tyres as it becomes necessary to save money..

For speed devices the method varies according to the device..basically there are self testing buttons, tuning forks,drive throughs by police vehicles with calibrated speedos, to name but a few..

However, remember a defective speedo is no excuse, in fact it is an additional offence, which I know is regularly enforced by way of prosecution,especially by the Camera Enforcement Office(s),for there is a degree of mens rea (guilty mind literally PT).

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