Speedo reading

[westbay]

Hi,

I have a 1997 2.2dt , very happy with it . I have noticed that he speed reading appears to be about 10% out (too high) throughout it's range - is this normal ?

It's wearing 195x65x15's which I think are correct from the handbook reference........

[Lennart Sorth]

speed reading appears to be about 10% out (too high)

My 2002 2.2dci is similar on its standard boots.

I think its a conspiracy (sort of) :-)

It only makes people drive slower than the speed limit (nobody can argue thats an unsafe thing), but more importantly, it may lead the the car actually reaching its promised fuel economy ...

I have seen this in a long range of modern Citroens and Renaults - maybe its a French implementation of political correctness ?

/Lennart

[roy4matra]

I have a 1997 2.2dt , very happy with it . I have noticed that he speed reading appears to be about 10% out (too high) throughout it's range - is this normal ?

The law states that a speedometer can be inaccurate by plus 10% to minus zero.  In other words it can read higher than what you are actually doing, but never less.  Most cars on the market no matter what country they come from, over read, and have done for many years (probably since they began!) which is one reason you can safely go past a speed camera a little over the limit on your speedo since in fact you are probably just under in reality!

It is also why some people think trucks are going faster than they are allowed - trucks use Tachographs that have to be calibrated far more accurately than the car speedo so when you /think/ you are doing say 55mph which is the truck limit (85kph) you are probably only doing 50mph and the truck going past you on his speed limiter is doing a true 85kph!  Since there are always small tolerances and tyre wear, there can always be slight differences too.

Roy

[Lennart Sorth]

[The law states that a speedometer can be inaccurate by plus 10% to minus zero.

Yes, that's correct - but personally I would like my car to show  max +1%  - as a physicist, I'm very opposed to be given deliberately false readings. 

That is also why I bought oversize summer-wheels for my Espace, which has given it approx 1% overrreading, which is more acceptable.   :-.)

That said,  my Murena has always been very precise, - never under-reading, but mostly been less than a few (3-4?) percent. So even old-fashioned, cable-driven speedos are capable of sufficient precision to enable such small overreadings.

I testdrove a Citroen C2 and a C3, when they just came out - and using GPS we measured both of them to do 88.0 km/h when the speedo showed exactly 100 km/h - that *is* pathetic, but I'm convinced it has to do with the maybe somewhat optimistic fuel-economy promises.

/Lennart

[westbay]

Lennart,

What size of oversize 'summer wheels' did you get ? that sounds to me a much better solution !

[renaultbiler]

Just to fill in, on BII Espace's supplied with Bosch ABS 5.3 with speed information you can alter the tyre dimensions with XR25 or CLIP to get the speedo as correct as possible - the ABS supplies the speed information to the BII. I think this goes for most models from -99 up. (NT3239A)

[westbay]

Good point but is there a  piece of kit which performs similar fiunctions i.e. allows the user to make changes to these peramiters as I believe the Renault CLIP is a) not available to enthusiasts and B) even if it was it would be too expensive !

I'm thinking of something like VAG-COM which I have for my  VW

[renaultbiler]

This does not affect your car as it is pre BII (no BII box in the floor) - and i dont know if any "cheap" tool can do this at all, but certainly other professional diagnostic tools may as it is a Bosch computer.

[Lennart Sorth]

Lennart,

What size of oversize 'summer wheels' did you get ? that sounds to me a much better solution !

Cannot remember off my head (and I'm currently not in the same country as the car) , but 17" OZ alloys with 225-55 sounds about right.   (EDIT 20080128 - 225/55/17 was in fact right, - standard is 225/55/16)

This is btw how it looks:
http://www.matrasport.dk/Cars/Espace/Gallery1/Med/je2.jpg.html

My winter wheels are now the original 16", which only has the effect that we possibly drive a bit "too slow" during winter, which cannot be a bad thing. We'd like to keep the car in one piece :-)

/Lennart

[TheJoker]

My speedo is very accurate around 30mph, at 50mph it's a bit slow (real speed probably 46mph), at indicated 70mph the car is probably doing some 64mph.... to get a real 70mph (UK motorway speed) I need to do 76ish on the speedo. This is just from my memory, and my accurate readings come from driving with a Garmin eTrex Legend C GPS.... that said, I've managed to get the car up to 115mph on the speedo.  (didn't have time to look at the GPS :p )

[westbay]

Thanks Lennart- do you trhink i should go  from 15's to 17's or 16's ? the handbook shows 15's but i see a lot of forums quoting 16's  :javascript:replaceText(' \', document.forms.postmodify.message);
Undecided

I checked my readings with  GPS and confirmed near enough 10% fast