getting realtime speed on MFD

[chrisboyle999]

i think its a bit more accurate than you are giving it credit for.
i got a ticket the other day and noticed when the camera flashed i was doing between 35-40mph on the gauge, the ticket said 36mph when it arrived.

admittedly it may get worse as the speed increases.

 

[stuartsjg]

The 'logging function'...that sounds interesting. Any idea how it can be set up on a Tom Tom, or were you speaking with regards to the SEAT standard fit unit?

Sorry for the confusion, by SatNav, im talking about a seperate unit, eg TomTom, Navman etc.

Most can log all the satellite data, my navman can log this every second of a journey. There is then software available which can read the data and recreate a journey, including speed.

People have successfully used this where, say a police partol car with an uncalibrated speedo logged you at, say 105mph which warrants loss of license, but the satnav data has shown that you were actually doing 93mph.

Provided you can prove the police used an uncalibrated speedo as a basis for the ticket, as opposed to a dedicated instrument, satnav or calibrated speedo with valid calibration, its probable that your offense would be down graded.

A ticket is only valid provided it shows a real life physical speed.

 

[stuartsjg]

i think its a bit more accurate than you are giving it credit for.
i got a ticket the other day and noticed when the camera flashed i was doing between 35-40mph on the gauge, the ticket said 36mph when it arrived.

admittedly it may get worse as the speed increases.

That would sound right, 30 + 10% + 2 = 35mph setpoint of the camera
(assuming its safe enough to travel sit on or just over the lower limits, use cruise to stop you creeping in 20's, 30's, 40's and itll save u a fortune in fines and points)

Your reading is quite realistic, the speedo is allowed to be 100% accurate, meaning real life speed. Its also allowed to be as much as 10% + 4km/h (or 5% + 10km/h by the uk standard)

The only way to know is to find a road which has calibration squares or use a satnav.
You can get racing/sports ones which update several times a second and measure to 0.1 or 0.01 mph. Also used for drag racing etc..

The squares are two solid white squares, around 1ft square which are placed exactly 1/2 mile apart, generally in a 60+ zone. Some poor guy would have measured it to the inch!

Police patrols use them to check the speedo error.

You maintain a fixed 60mph on the speedo (using cruise if available) start timing as you see the 1st square disappear under the bonnet... stop at the next square at the same point it disappears.

Taking into account your reaction time, this 1/2mile should take 30 sec if the speedo is right. Obviously give somebody else the stopwatch, or for best accuracy, a window viewing video camera so you can do frame/frame replay of the moments the square passes to get the exact time.

Then repeat the same thing at 30mph indicated speed. This should take 1 minute.

In real life, this will take longer than 30sec or 1 minute.

In your case, i would think if you do this at 30, you will find it takes 1 minute or just a sec or two longer. They do get worse as the speed increases, this is why you normally do the squares thing at 30 and 60.

Stuart

 

[phil3012]

i think its a bit more accurate than you are giving it credit for.
i got a ticket the other day and noticed when the camera flashed i was doing between 35-40mph on the gauge, the ticket said 36mph when it arrived.

admittedly it may get worse as the speed increases.

I got done on my Cupra as well, seem to remember the speedo read 41 and I got done 39

The gutting thing was apart from the 3 points and the £60 fine that it put our lasses premium up by £30 when she told them at renewal as I am a named driver on her car as well.

 

[Rampage]

The amount of inaccuracy on the speedo will depend on the current depth of tread on the tyres.

 

[stuartsjg]

Found a wiki article which covers the error caused by tyres:

percentage error = 100 x ("standard diameter"/"actual diameter" - 0)

so for, say my 225/40R18, their new diameter is 637.03mm

New the tyres generaly have 9mm of tread, minimum legal is 1.6mm, most people change around 2mm. Thats 7mm loss x 2 = 14mm off the radius.

so:

error = 100 x ((637.03/623.03)-1)
error = 2.25%,

the reading will be an additional 2.25% over true speed due to tyre wear of this degree.

Smaller tyres do more revolutions per mile, so the car things its going faster than it is.

What i dont know is, what figure for tyre diameter does the manufacturer program into the computer? The new diameter, or a halfway between new and worn? My guess is the new diameter. If they used the worn diameter, then new tyres would underread which isnt allowed.

Stuart

 

[phil3012]

Found a wiki article which covers the error caused by tyres:

percentage error = 100 x ("standard diameter"/"actual diameter" - 0)

so for, say my 225/40R18, their new diameter is 637.03mm

New the tyres generaly have 9mm of tread, minimum legal is 1.6mm, most people change around 2mm. Thats 7mm loss x 2 = 14mm off the radius.

so:

error = 100 x ((637.03/623.03)-1)
error = 2.25%,

the reading will be an additional 2.25% over true speed due to tyre wear of this degree.

Smaller tyres do more revolutions per mile, so the car things its going faster than it is.

What i dont know is, what figure for tyre diameter does the manufacturer program into the computer? The new diameter, or a halfway between new and worn? My guess is the new diameter. If they used the worn diameter, then new tyres would underread which isnt allowed.

Stuart

Interesting article, never realised the tread depth would make any difference. I would have thought they'd use the new depth or near to it and the law on min depth changes and 3 mm is the recommended depth to change at.

Does changing the size of wheels have an impact?

I know it does if you out bigger wheels and tyres on, but if the rolling circumfrance of the wheel stays the same (by adjusting the profile of the tyre to match) would that make a difference?

 

[stuartsjg]

Does changing the size of wheels have an impact?

I know it does if you out bigger wheels and tyres on, but if the rolling circumfrance of the wheel stays the same (by adjusting the profile of the tyre to match) would that make a difference?

Provided the rolling circumferance is the same then there will be no diffirence.

Ive heard of Vauxhaul Nova boy racers (i dont know any) having problems by putting on larger circumerfance tyres, they leave the suspension the same so bash the tyres on the wheel arch and also their speed registeres lower than it is actually.

 

[chrisboyle999]

speedometer readings are taken from the transmission. so while actual speed may vary slightly with tyre diameter, speedometer indication will not.

 

[stuartsjg]

speedometer readings are taken from the transmission. so while actual speed may vary slightly with tyre diameter, speedometer indication will not.

Yes, I think there is a hall sensor in the gearbox off the differential gears which is used for speed.
I also think that each wheel has its own sensor as in VAGcom you can pull up the km/h for each wheel. This is for the ESP, ABS and traction control.

What you say is true, but its also true the other way too.

At constant rpm in the transmission, the real vehicle speed will be less with a worn tyre but still indicate as normal.
At constant real vehicle speed, the transmission will need to run slightly faster with a worn tyre than a new tyre which means you get a higher indication.

Either way the error remains.

 

[chrisboyle999]

if i replace only one tyre on the front of my car and the other front one is well worn, will i go round in very large circles?

 

[Rampage]

speedometer readings are taken from the transmission. so while actual speed may vary slightly with tyre diameter, speedometer indication will not.

It will be the same won't it?
One revolution is still going to be different depending on the size of the tyre. ie One revolution is a different distance traveled.

 

[stuartsjg]

if i replace only one tyre on the front of my car and the other front one is well worn, will i go round in very large circles?

that would be fun if it did!

I think in nascar (or something like that) tyres on one side are diffirent to the other to help with the steering.

I think if there was a small difference the Diff would deal with it. Ive heard from tyre fitters that if you need a new tyre, and they are over a certain milage they must change a pair. Perhaps its to stop you driving in circles!

 

[stuartsjg]

It will be the same won't it?
One revolution is still going to be different depending on the size of the tyre. ie One revolution is a different distance traveled.

If your engine is at, say 2000rpm, the gears reduce it to some value (i don't know the ratios). The gearbox speedo takeoff will only see what speed the final drive of the transmission is turning at.

The speedo will be designed with a scale such that:

mph = transmission speed x pre determined conversion parameters
-and-
indicated speed = mph x allowable error​

Working it through both ways (constant real speed vs constant transmission rpm)
So, if you have 225/40R18's with a new diameter of 637.03mm
Circumference is just under 2m
At 70mph real speed your doing 31.3m/sec or 15.65 circumferences/sec or tyre 939rpm

Same tyre but worn to 2mm gives a circumference of 1.956m.
At 70mph real speed you need 16.01 circumferences/sec or tyre 960rpm.

With a worn tyre, the speedo is working off 960rpm as opposed to a new tyre of 939rpm.
As the speedo only knows mph = rpm x conversion parameter, your speedo will read higher with a worn tyre given your doing the same real speed.


Looking at the the other way...
If you turn a worn tyre at the same rate as a new tyre at 70mph real speed:
1.956m x 939rpm = 30.61m/sec or 68.5mph real speed.

if you turn a new tyre at the same rate as a worn tyre at 70mph real speed:
2.00 x 960rpm = 32m/sec or 71.6mph real speed.

Either way you look at it:
If you continue to go by the speedo as the tyre wears, your real speed will be gradually reducing

____
I think weve deviated off the line of the thread! Its probs best to keep this sort of talk hidden thought...

 

[Rampage]

Yes, thanks. I think I misunderstood the origional question.
What do you do for a job?

 

[stuartsjg]

Yes, thanks. I think I misunderstood the origional question.
What do you do for a job?

No probs. Affraid to say im an Electrical Engineer but im also invoved in alot of mechanical and fluid systems design and building too so theres alot of proplem soving to do!

Needless to say, people try and not let me start on this sort of thing in a pub... could be there a while!!

 

[priitpo]

As said before the speedo is programmed to give you higher speed than the car is really doing (the milage or km reading is still correct though) - but the car actually knows your real spped as well and this is really easy to check. Here's what you should do: Drive with a constand speed (really good to use the cruise control ), re-set your avarege speed indicator (on the mfd - doesn't matter if you have the big or a small one!) and check appearing numbers against the speedo dial!!!!

 

[craigomills]

So is it possible to get a speedo reading on the MFD?

 

[ChrisMansell]

not one that changes every millisecond like I presume you want, but one that displays your average speed. if you set your cruise control to whatever you want, reset the trip computer, look at average speed and it will have you actual speed.

so no, you can't get your real time speed displayed like the speedometer displays it.

 

[Rampage]

The above shows exactly the same speed as my sat nav.