S2 Speedo accuracy

[uk08462]

Whilst driving on the motorway yesterday, I was comparing the speedo reading of my S2 with that given by my Garmin SatNav.

At an indicated 50mph, I was doing just 47, 70mph was in fact 65, and (allegedly) 90 would have been 82. As I trust GPS rather more than a mechanical calculation, I am inclined to believe that the speedo on this S2 (at least) is inaccurate. Has anybody else noticed this as well or is it just my car?

Also, would this inaccuracy be reflected in the mileage?

[keithlard]

This is the case with almost all cars I've driven. Manufacturers tend to err on the side of caution (in other words the speedo usually reads slightly more than the real speed).

When I'm driving through a speed camera, I tend to err on the side of caution too

[uk08462]

I have a BMW 5-series which is virtually completely accurate, and a Volkswagen Touran which is not - it too reads higher than actual speed.

I really wanted to check if this was a known error or a problem with my car.

[tweetdriver63]

I saw a show on TV about this a couple of years ago. Basically, there are tollerances manufacturers have to follow, but some are more accurate than others. As I recall, BMW was the most accurate the tested. There are rules that say the speedo can't show LESS than your actual speed.

From what I see in my Elan, and what others here have said on other threads, you can expect most Elans to indicate about between 5 and 10% lower than a GPS will indicate. Mine is right at 10%.

[uk08462]

Then I shall feel safe when going through speed cameras then.

Colin Chapman always liked thinks to be 10% less - unless it was money!

[HJ2]

I believe the margin of speedo's was a bandwith of
zero to a maximum of 7% + 4 km/h
I've noticed that my S2 speedo is 4%, wich is not bad!

All deviations on the "+" side of course.
As a manufacturer you don't want law suits from people who get speedingtickets allthough their speedo was still indicating legal speeds...

[dapinky]

UK regulations allow for a 10% tollerance, but as others have alluded to, only on the +ve side, not -ve.

The 'accuracy' or otherwise can be affected by tyre pressures and tread depth (as well as obvious size differences).

I found thet mine was about 4mph out at 50 mph, but by removing and replacing the needle on the speedo, I have now got it at 49mph on satnav whilst indicating 50 (dead on at 30, and 2 out at 70) - so the 'accuracy' appears linearly inconsistant (if that makes sence).

[tweetdriver63]

UK regulations allow for a 10% tollerance, but as others have alluded to, only on the +ve side, not -ve.

The 'accuracy' or otherwise can be affected by tyre pressures and tread depth (as well as obvious size differences).

I found thet mine was about 4mph out at 50 mph, but by removing and replacing the needle on the speedo, I have now got it at 49mph on satnav whilst indicating 50 (dead on at 30, and 2 out at 70) - so the 'accuracy' appears linearly inconsistant (if that makes sence).

Mine used to be a lot worse, then I noticed when I had the cluster out (therefore disconnected from the speedo cable), that the needle didn't rest all the way down on the little peg. I pulled the needle off, removed the peg, replaced the needle where it would be below the peg, rotated it to put the peg back in, and found it to be much improved when I got it all back in. I didn't feel like pulling the cluster again, so I accepted it being 10% off. It is consistently 10% off, regardless of the speed I'm driving.

Since my electrical fiasco recently, I have to replace the fuel gauge voltage regulator on the back of the cluster, which means I have to pull the cluster anyway. When I do, I'm going to do another needle adjustment to see if I can get it closer.

[ektrolleyboy]

Mine's about 10Mph out when doing 30+...

Is there a fix for this?

[dapinky]

Mine's about 10Mph out when doing 30+...

Is there a fix for this?

Yep.....

.......book it into Paul Mattys

Seriously though, if you remove the instrument panel, then the speedo from the panel, then you can manually hold the needle at 70mph (or any other number you want to use, depending on what you want to be totally accurate as per satnav), look at the rear and make a mark with a CD marker pen on the rotating part and also the bit it lines up with.......

.... carefully pull the needle off the spindle, then looking from the back line up the marks you made earlier......

....replace the needle whilst holding the rotating bit from the back (lined up), making sure it is now pointing at the number you want it to.

Eg if Satnav says you are doing 70, but the speedo says 78, then make your marks when the needle is at 78, take it off and replace it pointing at 70.

It's actually dead easy to do

[Simon_P]

As Dapinky says it depends on tread depth.... New tyres have a greater circumference than worn ones. So the spedo should be most accurate with new tyres and read progressively faster as the wear.

[Blue_Bullet]

Mine always says I am going faster than i am!

makes my 0 - 60 times quicker lol and maybe saves me from the cameras!

Putting bigger wheels on would ?

1:) Make it look like you are going faster?

2:) Slower?

[HJ2]

So... your's is like the rest of em then!
Speeding camera's... see my post above
Bigger wheels = larger distance per rotation. Speedo measures rotation, not distance.... thus.... (drums please)
The bigger your wheels, the slower the speedo indacates @ a given speed

I'm afraid the 0..60 times are not much improving. in fact: bigger wheels mean a larger momentum (torque) is needed for accelleration. (mostly they are heavier as well unless you want to spend a fortune!) So, if you want to be fast: get smaller wheels. still... no tires in the world could deliver enough traction @ slow speeds... And stock performance is way enough to loose your licence... You do the maths

[Gazza]

Don't think bigger wheels make any difference to the speedo as you use smaller profile tyres, making the diameter of the wheel/tyre the same as before

Gazza

[HJ2]

Then the diameter would be the same, thus NOT bigger
Or do we have a language thingey here? (Wheel is everything that rotates, Rim + tire)

[Gazza]

The bigger your wheels, the slower the speedo indacates @ a given speed

15", 16" or 17" wheels all have the same profile, no one puts bigger wheels and bigger tyres on an Elan.

[GeoffSmith]

no one puts bigger wheels and bigger tyres on an Elan.

Someone might have!

[HJ2]

Mine always says I am going faster than i am!

makes my 0 - 60 times quicker lol and maybe saves me from the cameras!

Putting bigger wheels on would ?

1:) Make it look like you are going faster?

2:) Slower?

Sorry, i thougt this was a question.

[GeoffSmith]

Sorry, i thougt this was a question.

So did I!