ZX Speedo Error ????

[sooty]

Today I borrowed my Son's "Road Angel" (Snooper for speed camera's).

This works with a GPS system locating your movements from 4 out of 5 satellites, and its accurate, as such as TomTom SatNavs etc.

I found that speedo reading is approx 6% fast ie: speedo =70mph, GPS=64mph. - speedo =30mph, GPS = 27mph

Although this gives a saftey margin with the speed camera's and PC Blod with his Radar gun, but you are loosing out on Fuel Economy. Granted this such a small error, but when you do alot of mileage in a year, thats Money you are wasting.

I run on 185/60 x 14 tyres, from this I will look for another tyre size to remove this error. Then its up to me to keep my wits about me with speed I'm travelling at.

[AndersDK]

All cars must by legislation read a bit faster than actual speed - because it must never under any circumstance read slower.
Up to +10% faster than actual speed is a very common reading - and has been since the Vikings were around 8)

[jeremy]

You're not loosing out with fuel economy - it just is that your calculation isn't quite accurate!

Not very difficult to devise a spreadsheet to make the necessary corrections - of devise a constant to put in the calculation formula - but for super accuracy you are going to have to allow for the diference between new and old tyres.

[Mandrake]

A 6% error is far too much to try and compensate for just by changing tyre size anyway, unless the car has completely the wrong tyres on it now.

For example when I went from 185/65/15 to 205/60/15, they are only 1% bigger rolling radius. And a 195/60/15 is about 1% smaller.

A tyre that is 6% larger in diameter probably won't even fit in the wheel arch. By the way, 64mph from 70mph is 9% not 6%.

The "error" will be due to the speedo, not the tyre size.

Regards,
Simon

[Mandrake]

All cars must by legislation read a bit faster than actual speed - because it must never under any circumstance read slower.
Up to +10% faster than actual speed is a very common reading - and has been since the Vikings were around 8)

Hi Anders,

I've known for a long time that car speedos were only required to be accurate within 10% but I have never heard of what you say about reading too slow being illegal, at least not in New Zealand.

In fact until recently speed cameras here allowed for up to 10% over speed before ticketing for this very reason. (In a 100Km/hr zone you could do up to 109Km/hr before a ticket)

When they reduced this to 5Km/hr over the limit there were many complaints specifically for the reason that typical speedo inaccuracies could put you over the new threshold even if you were being law abiding by keeping your speedo at 100.

I think you'll find it depends on the country...

Regards,
Simon

[ssray]

on my last bike i fitted a mountainbike speedo, you measure the outside radius of the tyre in mm type that into the unit, it takes a reading from a sensor and a small magnet that you attach to part of the wheel.
i thought about putting the speedo in the space where the clock goes on my bx.

[RichardW]

UK Construction and use regulations specify +10%, -0% error AFIAK - most makers stick to nearer the +10 these days to be on the safe side. Interestingly the error usually only affects the speedo and not the odo part. My 95 Xantia TD was pretty close on speed, but my 97 is well over (reads about 83 at 3000 rpm, compared to 75 actual), but the odo is pretty well the same, reads 210miles door to door between our house and our friends' house. If the 10% error in speed was compounded into the odo, it should read somewhere nearer 230 miles.

I generally ignore the speedo and use the rev counter - 2000 rpm = 10,20,30,40,50 mph in the relevant gear - has served me well with no speeding tickets for the last 150,000 miles in TD powered Citroens!

[Homer]

UK Construction and use regulations specify +10%, -0% error AFIAK

Chapter and verse.

EU regulations are +10% +6.25mph at all speeds between 25mph and the vehicle's maximum speed, and never less than the true speed.

Note that below 25mph it merely has to read your true speed or higher (so could read 84mph at 24), same goes for anything over the vehicle's top speed.

[James.UK]

I don't see how the speedo reading fast can effect your mpg? Surely it's distance covered divided by fuel used to get mpg? Your speedo reading fast wont make the car use any more fuel..

I agree with Richard, the odo in most cars are pretty accurate..

The speedo in my 1.9D ZX is not too important really, apart from speed limits, a calender would probably be better.. lmho

[mezuk04]

I use TomTom on my handheld PDA and it displays my speed, I get the exact speed reading as you have encountered....and i trust the satellite more than the speedo and with no wheel size modifications.

So I now know im not going quite as fast as I was

[sooty]

With any speedo error reading fast, when you are suppose to travel one mile. You will infact only travel that one mile minus the error. So if your vehicle is suppose to have mpg of 40 miles to a gallon, you will get it, minus the error. So if you do alot of mileage over the year, that error in distance travelled mounts to money.

In other words if you travel over that same mile at a lower rpm, you save fuel over the year, especially where fuel is for ever increasing in cost.

[jeremy]

Strange - when I the speedo cable failed on my car it still continued to use fuel!

The amount of fuel used depends largely on the work done not on the engine revs. Using third when you should be using 4th will use a tiny amount more fuel to compensate for the small amount of extra friction in the engine and the pumping losses but that's it. Accellerate hard up a hill in 3rd with the turbo running will use loads more than maintaining an even pace, and you'll do better still if you allow it to gain some speed on the previous downhill and let the speed drop up the hill.

[sooty]

[Clogzz]

when the speedo cable failed on my car it still continued to use fuel!

Thanks for this enlightening contribution, Jeremy.