MPH accuracy?

assuming a straight level road and a steady speed then the satnav will be right. most speedos read high, which is why so many people travel quite a bit below the speed limit, particularly noticeable in 50 roadworks areas on motorways.

The average car speedo reads 7% high. Based on many tests of many cars over

Reminds me of the workshop manual for my old Subaru. When it came to testing speedo accuracy it said that a speedo showing 60mph at a road speed of 60mph was out of tolerance and must be replaced. It had to read between 61 and 64, or something like that..

John

I believe the tolerance to be +10% -0% (possibly +10% plus 6mph between

Think it is +10%/-0% at 30. Which is why all only read high.

It's a total nonsense in this day and age. The very cheapest pulse counter can easily better 1%.

The speedo on my SD1 is as accurate as you can read on an analogue type. Think Jaguar had pretty accurate speedos too. My much newer BMW reads 76 at a true 70. Designed to do so, I'd guess.

Obviously designed to read +10% but they fitted the wrong diff at the factory.

Er, 30 +10% +6mph is 39mph. I don't think you'll find many speedos overreading by that much!

The speedo on my Saab 9000 agrees completely with the speed on my TomTom. In fact I often get speeding warnings from roadside speed warning indicators when I drive at the exact speed. e.g. 50mph.

Yes. Certain makers fit (reasonably) accurate speedos, and others don't. And those who fit over-reading types are often very consistently wrong. Which means it was a design consideration.

Very good. But having owned the car for some 25 years, even I might have noticed that by now. ;-)

This is what Wiki says:-

European Unionmember states must also grant type approval to vehicles meeting similar EU standards. The ones covering speedometers [8][9][10]are similar to the UNECE regulation in that they specify that:

 The indicated speed must never be less than the actual speed, i.e. it should not be possible to inadvertently speed because of an incorrect speedometer reading.  The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. For example, at 80 km/h, the indicated speed must be no more than 92 km/h.

The standards specify both the limits on accuracy and many of the details of how it should be measured during the approvals process, for example that the test measurements should be made (for most vehicles) at 40, 80 and 120 km/h, and at a particular ambient temperature. There are slight differences between the different standards, for example in the minimum accuracy of the equipment measuring the true speed of the vehicle.

The UNECE regulation relaxes the requirements for vehicles mass-produced following type approval. At Conformity of Production Audits the upper limit on indicated speed is increased to 110 percent plus 6 km/h for cars, buses, trucks and similar vehicles, and 110 percent plus 8 km/h for two- or three-wheeled vehicles that have a maximum speed above 50 km/h (or a cylinder capacity, if powered by a heat engine, of more than 50 cm³). European Union Directive 2000/7/EC, which relates to two- and three-wheeled vehicles, provides similar slightly relaxed limits in production.

It also says the error is allowed to cope with variations in tyre sizes and wear. Which I don't understand - as a smaller tyre due to it wearing will make the speedo read high. A larger tyre than the design size would then cause it to under read, which isn't allowed.

I agree, having used several different GPSRs, and a SatNav and made the comparisons with three different cars.

The current Focus is only very slightly under the speedo reading.

However, it is NOT a good idea to look at a SatNav whilst driving!

My much newer BMW reads 76

... To allow for idiots who can't drive RWD cars.

(Present company excepted, of course) ;-)

There is an app for modern type mobile phones that shows your speed in reversed digits, if you put that on the dashboard you get a head up display reflected on the windscreen. Some of the newer gps units can also send their info to your phone so that you get a head up display of the driving directions too. Some modern cars have this as standard too.

Some pillock I know quite well left his Garmin GPS unit on the roof of the car to warm up a bit quicker so he could compare his speedo indications with the Garmin unit.

Waited a while just about to bring in inside the car then the phone rang long argument with someone forgot about the roof borne GPS unit and drove off;(..

Well I hope that someone found it and made some use of it if it was still in the one piece!...

How do you keep your tyre diameter that constant?

"Dave Plowman (News)"

But it is necessary to distinguish between production tolerance (which would have applied to the older type of speedometer, meaning that cars of the same model could have had indicated a spread of 10%) and to deliberate offset (which applies now, meaning that all cars show the same wrong value).

Interesting, brings my Flight Sim HUD into the car.

It wasn't me, because it was only put on the car roof whilst I was changing into my boots to go walking. ;-)

My latest Satmap gets a fix almost as soon as I switch on.