Talk Morgan Forums Community Vehicle Talk (non-Morgan) Why are speedometers so optimistic

Reminded of this issue by another post about speedometers I have a couple of observations and questions about overriding speedometers in general.

The speedometers on mo cars these days seem to over-read by 3-5%. Now i understand that by law they have to be optimistic but in this modern day and age why is the average margin so great. My wife's 1983 2cv is much more accurate and ove-reads by maybe 2%. If I drive a car with a standalone GPS in the car this gives the speed and I see quite clearly the over-read of the speedometer. Yet on a car with a built in GPS it doesn't show the GPS calculated speed and therefore I don't see the over-read of the speedometer; why?

In France, I believe in line with other European countries, HGVs are limited to 90km/h. When following trucks on a level motorway and measuring mine and their speed with a standalone GPS I've noticed that their real speed is within about 1% of 90km/h, so their speedos/governors are much more accurate than on cars..

This isn't a rant, and I am maybe not being very clear in my points. I just don't get why in this day and age the average car speedometer is so inaccurate? Marketing?

Cheers

Just repeating what I posted on another thread but UK law based on EU regs requires car manufacturers to provide speedometer accuracy in the range of 0 to 10% plus 6.25 mph over read in the true speed range 25 to 70 mph. It cannot read below the actual road speed therefore and the over read is limited to the formula is V/10 + 6.25 mph where V is the true speed.
This means a vehicle traveling at a true 40 mph could have a speedometer indicating 50.25 mph and still be legal

In reality most seem to read around 5% over I've found. Having said that my 55 year old Land Rover is spot on courtesy of 7.50 tyres

Well mine reads 10% under & MMC say that if fine.

So I guess the odometer might be reading 10% under too....

My speedo in the 4/4 is reasonably accurate, at least until I've covered a few miles, then it could read anywhere from 10+mph over or under, inconsistently. The mileometer on the other hand under reads by some 15+%, so the car has travelled further than displayed. Only discovered the under read on a trip home from Aylesbury a couple of years ago, so whether it has always done it is anyone's guess.
The 90, courtesy of 7.50R16 tyres over reads speed slightly up to 40mph and then under reads a little from around 50mph, so displayed 60 is in reality 62mph The original 205R16s gave more than 10% over, so I resorted to a GPS based add on speedometer.

Well MMC are wrong ! you can re-calibrate it using the reset button, there is a thread on here somewhere on how to. Might have been a Gambalunga post.Mine was done by Melvyn.

Check out Gambalunga's post 12/11/2016 it tells you how to re-calibrate the speedo.

Richard, I understand about the law but why such big over-reads. Speedos could certainly be more accurate, witness the 2cv. Also why stop in-built GPS devices showing true speed?

One of the reasons it's getting to me at the moment is that living in France I do a lot of driving on N & D roads that are most of the time limited to 80km/h (though some departments are 90km/h). Lots of drivers understandably are wary of all the speed cameras and so drive along at an indicated speed of just under 80km/h which, is often a true speed of a little over 70km/h. I'm all for safety but such low speeds on good N & A roads are crazy and in fact some times causes people to overtake dangerously because they find the low speed unacceptable.

Cheers

Than it looks like your car needs less fuel.

Stefan

it is different on the roadster . The instrument they say can't be recalibrated. It has the famous MMC chip to stop revhang & that is locked. the odometer seems to read fine. As the speedo sits in front of the passenger & under the tonneau cover I fitted a GPS speedo above the wheel. That is pretty accurate & I can see it