Speedo Over Reading

[roy666]

I've only just realised that the speedo in my S2 is over reading by about 6-7 mph. No mods on the car, normal wheel and tyres etc. How does one calibrate the speedometer?... I also have an rpm dial that fluctuates upwards (every now and then) when in fact the engine is not screaming up to 4500 rpm at all... but thats another story! Is the speedo an simple job? thanks all!

[tweetdriver63]

A few things helped mine. First, I realized that my speedo read about 10% too high, so I just made a mental adjustment. Next, when I was doing some work on the cluster (refinishing), I replaced the needle oriented a bit more counter-closkwise (anti-clockwise, if you prefer) then held it out of the way while I put the stop peg back in. That corrected about half the error. Finally, I got some Metal Monkey dial faces last year. They have them made up with the original markings to 170mph, but because our cars are pretty famous for being optimistic on the speed, they also have them calibrated to 160mph. I got the 160 face and now my speedo is almost dead-on.

[HJ2]

When i replaced the dial faces i left the speedo indicator needle of, went for a spin on the motorway and used the GPS to drive a steady 100 km/h.
Then i stuck it back, indicating this. Worked flawless!

[tweetdriver63]

When i replaced the dial faces i left the speedo indicator needle of, went for a spin on the motorway and used the GPS to drive a steady 100 km/h.
Then i stuck it back, indicating this. Worked flawless!

You stuck it on WHILE you were going 100 kph? Seems a little daredevilish to me. haha

[HJ2]

Yep, it was easy on the straight, empty road.
(but you could do it at 50 i suppose, haha )

[lotos]

I seem to think that every vehicle I've had since GPS came out with an analog speedo seems to be 10% fast. That's on a Lexus SUV, a Suzuki scooter, an Audi car, and a few others I can't remember. All of them almost exactly 10% fast on the speedo compared to GPS. So I know I can go a little bit faster without Johnny Law chasing me down. I have had two cars with digital speedos (currently a Nissan Leaf) and they were both spot on. I have to remind myself all the time that it's accurate so I don't inadvertently speed.

So is it something that just is within build tolerances for analog speedos maybe? Or maybe they do it a little on purpose to keep you within the law. I know that on many luxury brands they lie about the gas gauge (showing it empty too soon) because nobody wants to be stranded on the side of the road in their new Cadillac..

[dapinky]

It is fairly easy to change the needle position on the shaft...

As others have said, the trick is to know where to put it - I drove at 60mph (on the satnav) and noted what the speedo was reading (about 55ish IIRC).

Then once i had got home, remove the speedo from the instrument panel - hold the needle on the 55 as indicated, hold the backing disc still, pull off the needle and replace it at 60 mark, then let go of the back disc and needle falls back to 0.

Refit speedo, go for a test drive - mine was now accurate at 60mph (but if not, just repeat until you're happy with where it is).

Job done!

It is slightly out at other speeds (but not as much as it was before i started), and I wanted accuracy at a 'usable' speed - but there's nowt to stop you choosing where you want the "accurate" speed on the scale - it isn't a totally linear scale, but is a hell of a lot better than it was before.

[tweetdriver63]

I seem to think that every vehicle I've had since GPS came out with an analog speedo seems to be 10% fast. That's on a Lexus SUV, a Suzuki scooter, an Audi car, and a few others I can't remember. All of them almost exactly 10% fast on the speedo compared to GPS. So I know I can go a little bit faster without Johnny Law chasing me down. I have had two cars with digital speedos (currently a Nissan Leaf) and they were both spot on. I have to remind myself all the time that it's accurate so I don't inadvertently speed.

So is it something that just is within build tolerances for analog speedos maybe? Or maybe they do it a little on purpose to keep you within the law. I know that on many luxury brands they lie about the gas gauge (showing it empty too soon) because nobody wants to be stranded on the side of the road in their new Cadillac..

I saw a story on one of the 60 Minutes type TV shows a few years ago about speedometer accuracy. They tested a lot of different makes. Government regulations require a certain accuracy tolerance, and they can read faster than the car is actually going, but not slower. BMW was either the most accurate, or least accurate of major car makers (can't remember which). Anyway, it was an interesting show.

[HJ2]

It is all for legal reasons.
As a manufacturer you don't want to be held liable for speeding tickets as a result of a badly calibrated speedo

I think to be on the safe side (for the car manufacturer) the guideline is +7% + 3km/h

[Jeemy]

I once was told about a speed camera that had no film in it. Spent some time looping round and driving past it until it finally flashed at 77mph.

For years I thought that that meant you have 10% luxury on speed cameras. Now I realise the car was probably 10% underquoting the speed and you have much less tolerance.

I am sure somebody here will know the facts. Ah, the folly of youth.

[GeoffSmith]

On my spare instrument panel, the speedo points at the bottom of the P in MPH without the end stop.

It is all for legal reasons.
I think to be on the safe side (for the car manufacturer) the guideline is +7% + 3km/h

UK law is +10%+6¼mph/-0 and is based on EU law.
Don't forget that tyre wear will account for a few percent as well.

[HJ2]

On my spare instrument panel, the speedo points at the bottom of the P in MPH without the end stop.

It is all for legal reasons.
I think to be on the safe side (for the car manufacturer) the guideline is +7% + 3km/h

UK law is +10%+6¼mph/-0 and is based on EU law.
Don't forget that tyre wear will account for a few percent as well.

There you have it!

[lotos]

Don't forget that tyre wear will account for a few percent as well.

Thankfully in the right direction (making you think you're faster than you are)...