Technical Speedo Accuracy

[John R Smith]

Folks

Has anyone managed to check the accuracy of their 500's speedo yet, against a reliable GPS or whatever? Unfortunately road tests never give this information these days. I was thinking of figures for 30, 40, 50, 60 and 70 m/h.

Thanks in advance

John

 

[Jake_in_notts]

Ive noticed mine is reading low by almost 5mph at 30 50 and 70 read against my TomTom.

 

[Tourandot]

Mine's also around 5mph optimistic against both TomToms - i.e. 70mph on the clock = 65mph on the TomToms (I have a One XL and Mrs. T has a One V4).

It's closer than my Touran, which is a good 10% out.

I tend to rely on the TomTom at speeds over 50mph, and on the speedo when there are cameras or in 30mph or 40mph zones.

T

 

[nottsfiat07]

So are we saying that sat navs are completely accurate on speed readouts then? Im not doubting it if so, just wondered if they were actually that accurate. Maybe someone out there has some info?

 

[pearce_jj]

in your touran you can bring up the true car speed on the climatronic, if fitted

 

[Tourandot]

So are we saying that sat navs are completely accurate on speed readouts then? Im not doubting it if so, just wondered if they were actually that accurate. Maybe someone out there has some info?

Not necessarily - just commenting that my 500 reads 5mph slower than two TomToms, and my Touran is consistently 5% out against my TomTom (not tried Mrs. T's in the Touran yet).

The reason I do speed cams on the speedo and not the SatNav is that I read somewhere once that car speedos were designed to read 5% out, so I figure if I go through a 60mph camera at 60mph on the speedo, I'm running at a road speed of approximately 54mph, and therefore unlikely to get snapped.

I'd expect my SatNavs to be reasonably accurate speed-wise, though the ETA I find is generally 4-minutes pessimistic, and the maps are about 10-yards behind actual road position.

T

 

[Tourandot]

in your touran you can bring up the true car speed on the climatronic, if fitted

I don't have climatronic on my Touran.

Climatronic would have been nice, but mine was a UK stock model in the right trim and colour with all but one of the options I needed - at £1000 off and an addition to the family imminent, it had to be done.

 

[John R Smith]

OK, thanks for your replies which confirm my own findings. I have checked the speedo on my 1.4 Sport using a Racelogic PerformanceBox on our local dual carriageway, and the results are that the 500 speedo is reading consistently fast, as follows -

True speed -30 m/h, indicated 32. True 40, indicated 43. True 50, indicated 54. True 60, indicated 65. True 70, indicated 75. True 80, indicated 86.

The rev counter seems to be spot-on accurate. I must say I find the results very disappointing, as with electric speedos there should really be no reason at all for the thing to read so far out. My old Jag read no more than 2 m/h out at 80 with an old-fashioned cable drive speedo. Incidentally the Racelogic box samples at 10 times per second and should be accurate to within 0.1 kph. I had full satellite lock-on at all times.

John

 

[Jake_in_notts]

My guess would be that setting the speedo to read concistantly high by 10% would allow for variables such as tyre wheel diameters and other tollerance to never make a speedo read less than the real m/h. There would be a lot of angry drivers around if they got tickets for sitting at 30 by thier speedo but getting snapped by speed cameras constantly.

 

[John R Smith]

Yes, that is precisely why it is done by the manufacturers. In fact there is an EU directive on the limits of error allowed. However, my point is that with electric speedos which are controlled by the body computer and CAN, there would be no problem in re-calibrating them via the OBD port for any wheel and tyre combination installed by the user. Tyre wear is not a problem, as it introduces an error of less than 2% at maximum. With today's technology, the speedo could (and should) be spot-on accurate.

John

 

[Jake_in_notts]

my deeler couldnt fit the battery terminal properly let alone calibrate a speedo to <1% accuracy

 

[trissybabes]

Ironically, when we used the Tom Tom One in any of our 3 cars (a Ford Fusion 1.4 TDCi, a Ford Probe 24v and a Ford Capri 280), it's the 20 year old Capri's speedo which is the most accurate!

The Fusion and Probe were reading about 10% higher than their actual speeds.

I think most cars have a 10% discrepancy in their speedo readings and the Police apparently allow for that - or so I'd heard...

 

[John R Smith]

The ACPO guidance to traffic police is that the motorist should not be booked until he/she reaches or exceeds the speed limit + 10% + 2 m/h. So in the motorway 70 limit you would have to be doing 70 + 7 + 2 = 79 m/h before you would be nicked. Hence having an accurate speedo is actually quite important, because you can cruise at 78 m/h and not be booked. And if you can, why not?

John

 

[Roadsterchinsk]

And if you can, why not?
John

Im not one of the speed kills brigade so ill not get into that - im more of a bad habits, not paying attention and inapriopriate speed kills.

Anyway, I read recently that with the high price of petrol and clear reduction of speed in what must be a economically minded driving population, they estimate based on previous figures that road deaths were down 140 people in the month of September in England and Wales due to a reduction in average speeds.

So given your question id suggest that a good reason is that speedo's that lie to us are actually saving lives. Seems like a pretty good reason to calibrate them lower to trick people into thinking they are already going the limit.

Im not going to get into all the pro's can cons of the current speed limits etc. Lets be honest we all see terrible driving by some on a daily basis (metalic red 523i on M11 last night the dotted lines are there for a reason and that's why its called a lane and you have to decide on which one and Aprilla coming off the A406 onto the redbridge roundabout this morning that dotted line is not a special motorbike lane and you do not always have special permission to pass even when there is no space) its pretty clear that if speedo's were accurate or people knew they could drive at 78 then we would have more accidents and more deaths - mainly due to the minority who treat driving nonchalantly.

 

[Venters]

The ACPO guidance to traffic police is that the motorist should not be booked until he/she reaches or exceeds the speed limit + 10% + 2 m/h. So in the motorway 70 limit you would have to be doing 70 + 7 + 2 = 79 m/h before you would be nicked. Hence having an accurate speedo is actually quite important, because you can cruise at 78 m/h and not be booked. And if you can, why not?

John

Guidelines are simply that guidelines.
The law is very different. Just last week a friends wife was done for speeding. 32mph in a 30mph limit.

 

[John R Smith]

Just last week a friends wife was done for speeding. 32mph in a 30mph limit.

Beast, that is one mean policeman. I would be outraged if it were me. All the more reason to have an accurate speedometer.

Let me be clear - I would never advocate breaking the law within urban posted speed limits of 20, 30 or 40 m/h. But 2 m/h over is something that happens to everyone, everyday, because you simply cannot drive watching the speedometer at every moment. Not if you are driving safely, anyhow.

John

 

[Venters]

Yeah to say she was surprised would be I think an underestimate.

 

[pearce_jj]

following the logic that 32 would have been showing near 37, which should be easy enough to spot

 

[pearce_jj]

Hence having an accurate speedo is actually quite important, because you can cruise at 78 m/h and not be booked. And if you can, why not?

well either by a tomtom, a capri 2.8 or a VW with climatronic. Sorted.

The capri gets my vote

 

[grimwau]

We have a "measured mile" in Canford Cliffs Road, Poole that the police techs used to use for calibrating the auxillary speedos in their cars but every time I go down the road I forget to look for the markers until I have passed them. :doh: