Speedometer Accuracy

[igdirect]

I treated myself to a Road Angel recently and immediately noticed that the GPS speedo in the device consistently reads 5mph lower than is shown on my GS300's speedo - and that's at all speeds from 10mph to 90mph

I also noticed that when going past speed cameras, all cars in front slowed down to the posted limit, for example 40mph (no doubt shown on their speedo as 40mph, as well as mine) but Road Angel indicated 35mph.

Has the over-cautious road safety lobby got at the manufacturers, I wonder?!!

Anyway, here's to life at 35mph in town...

Cheers!

Ian

[mudzs]

my road angel reads 80mph when my speedo reads 90mph.but mine gets further out the faster i go so at 100mph on the road angel my speedo reads about 115mph .i think the 18" wheels on mine causes this but would not have expected it with the smaller wheels ,what size do you have on yours

[greve]

All speedos show more than your actual speed. This is due to "speed kills lobbying" or similar. I've been told that a speedo must NEVER show less than the actual speed, so all manufacturers go on the safe side and generally show some 5-10% more than actual speed. This is good for avoiding speeding tickets...

And yes, new wheels/tyres in a different size than original influences the speedo reading. In the country where I learned about these things, only 5% difference in new wheels rotating distance from original is allowed.

[igdirect]

Hi Mudz,

Standard 16".

I think Greve has the answer here, though!

Ian

[mudzs]

Yep i agree the bigger the wheel the more out it gets ,how are you finding the road angel

[AltezzaScott]

I thought all speedo`s were 10% out by law??

[Kremmen]

As far as I understand it the telephones on Motorways are exactly one mile apart.

If you get the opportunity to travel a fair distance you should be able to hold a steady 60mph and see if it takes exactly one minute (or not).

It should then be possible to obtain a speed that = 60 secs between telephones and note the speedo ?

[GrandAlf]

I always thought that motorway phones were 2 miles apart. This was so you did not have to walk more than a mile to the nearest one. Maybe I am wrong?

[ColinBarber]

Yes speedos in the UK are made to read over. In Australia they are more accurate.

If you have Sat Nav (not IS) you can do a test. Pull up the fuel computer, set your speed using cruise control and reset the average speed. A few seconds later you will get a more accurate reading to compare against your speedo.

However even the tread wear on the tyres could make that 2 or 3 mph out.

[LexusEdinburgh]

Manufacturers set speedometers to read 5% more. I think they are permitted a +- 5% but to avoid litigation from people getting caught speeding, they opt for +5%. You are right about wheels and tyres. Any change in the rolling circumference of your tyre will distort the reading.

I have fitted Road Angels to some of my demos and all of my loan cars, not just out the kindness of my heart, I recently started to supply and fit them. So if anyone wants one fitted properly, you know where to come :winky:

Ian

[NickF]

I have noticed this as well. Why is it though, that when I follow Volvos on A roads, they always seem to be doing 47MPH, whichever speedo I am using? :P

[igdirect]

Hi Mudzs,

Road Angel is pretty good and superb for night driving when the cameras are not that visible from a distance. Best of all is picking up stray laser from roadside "safety vehicles"!

I now use it as my speedo, of course...

Ian

[stevenage_saint]

I seem to remember Top gear talking about this a few years ago. THey used a ford mondeao and had set up a system that uses what the Gatzo's are calliberated with. At the speedo reading of 30mph the car was travelling at 33MPH, at 40 on the speedo the car according the Calliberation tool was actually doing 45MPH.

The odd thing is when the speedo said the car was doing 70MPH it was actually doing around 63MPH.

The conclusion was the Gatzo's are too acurate compared to the speedos in the car. Funny thing is not many people have picked up on this.

Nick when I drive the Volvo it does not do 47mph on the A roads