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[JesterRT]

Dunno how accurate speedo's are, but, if I reset the average speed reading on the MID then I get told I'm doing about 2mph less than the speedo reads.  Eg. If I do 42, then MID reads 40.  Seems to get worse as it goes up.  

How do plod calibrate their speedo's?  It'd be nice to know that doing 70 means 70.

Can Tech2 calibrate them?

[TheBoy]

Dunno how accurate speedo's are, but, if I reset the average speed reading on the MID then I get told I'm doing about 2mph less than the speedo reads.  Eg. If I do 42, then MID reads 40.  Seems to get worse as it goes up.  

How do plod calibrate their speedo's?  It'd be nice to know that doing 70 means 70.

Can Tech2 calibrate them?

Is your speedo calibrated?

Normal ones tend to read slightly high on most cars by design.  The speedo will always have small inaccuracies, and remember the tyre diameter will change as tread wears and similar factors.

They cannot be calibrated via Tech2...

[JesterRT]

Tractor - not a chance that it was ever plod.  Guess my tyres are probably about 3/4 the way through their life.

Just wondering really.  I drive on the M42 and through roadworks on the M6 every day and wonder why so many people pass me - wish they'd use those damn cameras!

[Dave-C]

The way I see it, the calculation is the rolling circumference of the tyre, divided into a distance. The callibration companies have sophisticated stroboscopic rolling roads for setting them up...  The plods use the square markers on the carriagway to check the callibration each day on patrol....  also there are 16 marker posts to the mile on the motorways...  Locally to me , there is a tacograph install / service centre, VDO Kienzle... they have a strobe rolling road there,, watched it working once... Dont know how the adjustment is carried out on the Omegas though..  Anyone??

DC

[Kevin Wood]

I had to go through speedo calibration with the kit car at the SVA test. They run it on a rolling road at various speeds and compare readings. Don't know what they'd have done if my speedo wasn't connected to a driven wheel because they only had a single axle rolling road.

Anyway, the engine was in a shocking state of tune and it was quite amusing watching the giuy trying to keep a constant 50 MPH with no load on the engine and the exhaust and inlets popping and banging like a good'un!

The best way to check it yourself is probably to compare against a GPS receiver. GPS speed is pretty accurate as long as you set it up at a constant speed on a nice straight road and let it settle before making a comparison.

I doubt it ever gets calibrated for each individual car. I would imagine a value of pulses per mile from the sensor is calculated based on the tyres that go on it, with a bit of leeway to ensure it reads a little high, and the dash is programmed with this based on the model it goes in.

I'm not sure if the Police calibrated speedos are actually adjusted. It's probably simply that they test it to ensure it reads within specification periodically. Our boys in blue will probably confirm or deny this but I would imagine they'd use VASCAR or another gadget when it comes to handing out tickets and only use the speedo as a guide.

Kevin

[Marks DTM Calib]

The Police speedos are a different animal to the standard ones...they have extra graduations and only go up-to 150mph....they can be adjusted electronically...

In fact they are done in Nottingham!

[Joneyentee]

When I went down to Norwich to pick up my Omega I had a friend come with me and he brought his TomTom along. Driving down in my Shitreon the speedo read about 9 mph over what the gps said, but coming back in the Omega there was only about 2-3 mph difference. So from my experience it seemed spot on, once you take into account the leeway to make it read a touch high.

[Dave-C]

When I went down to Norwich to pick up my Omega I had a friend come with me and he brought his TomTom along. Driving down in my Shitreon the speedo read about 9 mph over what the gps said, but coming back in the Omega there was only about 2-3 mph difference. So from my experience it seemed spot on, once you take into account the leeway to make it read a touch high.

That'll be the rotation of the earth

[Joneyentee]

The Earth was rotating in the opposite direction to my driving, so it must've been passing under my wheels quicker. That would explain why the trip back in the Omega seemed quicker....  

[Markjay]

Find a road with mile stones, then use the stop watch on the MID to see how long it takes to drive through exactly 2 mile stones at a steady 60mph (cruise control will help here if you have it) then do the maths... one mile should be enough, but in principal the longer you go the more accurate the result. a motorway at night is best for this exercis. Back in the eighties it was 'common knowledge' that all speedos are calibrated at the factory to showing slightly higher speeds (up to 10%) so that the driver is not caught out - don't know if this still applies these days.

[Gwilym]

Back in the eighties it was 'common knowledge' that all speedos are calibrated at the factory to showing slightly higher speeds (up to 10%) so that the driver is not caught out - don't know if this still applies these days.

It is still the case and it has the added effect of making you think you get more MPG than you actually are.