Is 80kph really 80kph?

girl in the striped socks

So is it? Or is it nearer to 75kph?

whomitconcerns

girl in the striped socks wrote: »

So is it? Or is it nearer to 75kph?

80kph is 80kph.....

its nearer to 75kph than it is to 25kph....if that helps

Kenny Logins

Drive for an hour at 80 kph and tell us where you end up.

Zascar

https://www.youtube.com/watch?v=Qhm7-LEBznk

im invisible

nearer 100 tbh

Cunning Alias

Do you mean when it says 80kph on your speedo? In my experience, no.

I have a heads up display that gives me my speed in the same way a satnav does. If I am ever on a road where there is a speed sign (a sign that tells you how fast you are going), the value on my HUD and the sign will be the same, but my speedo will be higher.

E.g. If I am doing 50 on my speedo, the sign and HUD will say something like 45.

mgbgt1978

I believe you will find that 80kph really is 80kph....


....however, if you asking if you are really moving at 80kph when your speedo says so then the answer is usually No. Most speedo's over read by a margin of around 5%.
This is to allow for Tyre wear and also it helps car manufacturer's to avoid getting caught up in messy "I was only doing 79kph not 85, because my speedo said so" court cases.

Pilotdude5

Its 43 Knots.

KTRIC

Speaking as a biker I know the clocks on most bikes are purposely dialed in about 6% slow. I installed a "speedohealer" which lets me change the out put of the clocks. I've calibrated it to my GPS and its now bang on.

girl in the striped socks

So I may not be speeding if I was doing 85-90kph? Going by the speedometer on my car.

blindsider

I thought the HUD and the Speedo should read the same - after all they're displaying the same info from the same source.

It is not uncommon for Speedo's to be +5-10%, but sat-nav's are accurate to a few metres per second - that's how they tell you to turn in 300 metres - if they were out by more than a few metres, the whole technology wouldn't work.

On the Motorway, I set my cruise-control by the sat-nav - when the satnav says I'm doing 125km/h, I set the CC and leave it.

I have yet to meet a Guard who wold do me for being 4.1% over the speed limit and I'll take my chances with the vans - I reckon I'm safe there too.

djimi

girl in the striped socks wrote: »

So I may not be speeding if I was doing 85-90kph? Going by the speedometer on my car.

If you were doing 85-90 then you were almost certainly over 80kmph.

GPS devices and what not will usually return a reading of slightly less than the speedo in the car; in my experience its usually about 5kmph or thereabouts lower. However, its not foolproof either (youre basing your speed on a reading bounced off a satellite 10000 miles in orbit...) so if you want to be certain that youre under the speed limit then drive your cars speedo and dont try and work out some other figure; the cars speedo will never tell you that you are going slower than you are.

djimi

blindsider wrote: »

On the Motorway, I set my cruise-control by the sat-nav - when the satnav says I'm doing 125km/h, I set the CC and leave it.

I have yet to meet a Guard who wold do me for being 4.1% over the speed limit and I'll take my chances with the vans - I reckon I'm safe there too.

Just out of curiosity why risk it? 5kmph is no difference in reality, and at least if youre driving the cars speedo there is absolutley no chance that Gardai or speed van is going to clock you going over the limit. Why not just set the cruise control to 120kmph by the car?

dgt

Depends on the vehicle. My 406 reads 80, when it's doing about 75. The Daily van reads 80 but is doing 84.

sebastianlieken

Pilotdude5 wrote: »

Its 43 Knots.

it's also 22.22 meters per second.

Cunning Alias

blindsider wrote: »

I thought the HUD and the Speedo should read the same - after all they're displaying the same info from the same source.

My HUD gets it's speed like a GPS. It does not plug into the OBD port. I wanted to use one that did this but it set off a constant blinking maintenance light as soon as anything was put into the OBD port.

jack presley

I don't own a satnav so I was wondering is there a signal strength indicator on the device or does it let you know the number of satellites being used to calculate your speed/position at that time?

It's all well and good taking the manufacturer's accuracy claims as gospel but if there are only a couple of satellites orbiting above your postion at that time, is the output accurate?

Zubeneschamali

djimi wrote: »

5kmph is no difference in reality

It's about 12 hours a year, in my case.

Cerco

It all depends on where you are relative to the moving object. Einstein's Theory of Relativity applies. So it all depends.

shane.

Garda speed vans are set at plus 10% plus 2,so if it's an 80 zone and your doing 90 it's doubtful you'll get a fine

Carraig Fhearghais

Do the sat-nav readings take into account the gradient the car/bike is travelling up/down? AFAIK this may also give a variation if the satnav gives a 'flat' speed (as if travelling on a map).

just curious!

goz83

djimi wrote: »

youre basing your speed on a reading bounced off a satellite 10000 miles in orbit

They're actually only a few miles up, not even in double figures.

Carraig Fhearghais wrote: »

Do the sat-nav readings take into account the gradient the car/bike is travelling up/down? AFAIK this may also give a variation if the satnav gives a 'flat' speed (as if travelling on a map).

just curious!

As it is with everything, the more you pay, the better it is. But I don't think the sat nav in the car will take gradient into account. I could be wrong on this, but I believe the technology is simply longitude and latitude based and does not take into account the ground level. Some gps units would have this capability, for hiking and stuff.

Carraig Fhearghais

I thought I read that the satellites for GPS were around 12,000 miles up (medium earth orbit), single figures (miles) altitude wouldn't clear Mt Everest by too much!

Esel

goz83 wrote: »

They're actually only a few miles up, not even in double figures.

Doh!

goz83

Carraig Fhearghais wrote: »

I thought I read that the satellites for GPS were around 12,000 miles up (medium earth orbit), single figures (miles) altitude wouldn't clear Mt Everest by too much!

Apologies. I was thinking of something else when I was writing about satellite altitudes. I think the average satellite altitude is around 8k miles from sea level.

Google is your friend for these questions. Some satellites fly very high

http://www.spacetoday.org/Satellites/SatBytes/SatAltitudes.html

kirving

The GPS should take the gradient into account, there's no reason it wouldn't since it just calculates the difference between two points. If I was up a mountain, and the GPS disregarded my altitude and dropped me back to sea level, my lat/long would also be thrown off.

Debs_Mann

Some cars are near enough accurate to the speedo and speed you are travelling, i had a nissan qashqai hire car in april and it was about 10KM/h when comparing to speed on Sat Nav. My own car is about 2km/h off when compared to sat nav

cjt156

The satellites used for GPS are at Medium Earth Orbit - about 22,000 kilometers.
The Astra cluster, for satellite TV, would be in higher Geo-Stationary Orbit at about 35,000 kilometers.

keithclancy

djimi wrote: »

If you were doing 85-90 then you were almost certainly over 80kmph.

GPS devices and what not will usually return a reading of slightly less than the speedo in the car; in my experience its usually about 5kmph or thereabouts lower. However, its not foolproof either (youre basing your speed on a reading bounced off a satellite 10000 miles in orbit...) so if you want to be certain that youre under the speed limit then drive your cars speedo and dont try and work out some other figure; the cars speedo will never tell you that you are going slower than you are.

GPS Devices don't transmit anything, they are totally passive so there is no bouncing involved.

While they will be intermittently wrong they will not be consistently wrong.

http://airandspace.si.edu/gps/work.html

Yakuza

goz83 wrote: »

They're actually only a few miles up, not even in double figures.

Err, that would be commercial jets that are a few miles up. Satellites have to be thousands of miles/km up or good old gravity will just reel them in.

Edit - I should probably have read the rest of the thread before posting (then I'd have seen the correction), but the egregiousness of the error in the statement commanded my inner nerd to reply immediately!

jack presley

The Satnav's accuracy can only be as good as the information it is getting from the satellites which depending on where you are and what time of the day it is, will vary considerably.

I don't know anything about Satnavs but I have used GPS for surveying and know that to get an accurate reading you need at least 5 satellites in view to triangulate your postion accurately, which of course you don't always get.

Obviously the accuracy required for surveying is much higher and I know you don't need the same for Satnavs but can anyone tell me the number of satellites you need to achieve the +/- 5% accuracy on your speed and whether the devices let you know how many you have at that time?

dtipp

blindsider wrote: »

I thought the HUD and the Speedo should read the same - after all they're displaying the same info from the same source.

It is not uncommon for Speedo's to be +5-10%, but sat-nav's are accurate to a few metres per second - that's how they tell you to turn in 300 metres - if they were out by more than a few metres, the whole technology wouldn't work.

On the Motorway, I set my cruise-control by the sat-nav - when the satnav says I'm doing 125km/h, I set the CC and leave it.

I have yet to meet a Guard who wold do me for being 4.1% over the speed limit and I'll take my chances with the vans - I reckon I'm safe there too.

+1
I do the very same.
120km on my satnav shows about 114km on the speedo.
So I set cruise control for 123km/h (on the satnav).

28064212

Kevin Irving wrote: »

The GPS should take the gradient into account, there's no reason it wouldn't since it just calculates the difference between two points. If I was up a mountain, and the GPS disregarded my altitude and dropped me back to sea level, my lat/long would also be thrown off.

Say you have to travel to a point which is a straight-line distance of 100 miles away. In one case, it's a flat surface. In the second case, you have Mount Everest in the middle. As far as GPS is concerned, the distance you have travelled is 100 miles in both cases.

Alternatively, say you're in a lift on the ground floor (with an open roof so the GPS satellites can see you ). Then the lift takes you 500m straight up. As far as the GPS is concerned, you've travelled 0m, your lat/long coordinates are exactly the same

daveyboy_1ie

shane. wrote: »

Garda speed vans are set at plus 10% plus 2,so if it's an 80 zone and your doing 90 it's doubtful you'll get a fine

Is this true though? I heard this years ago myself, hell I even tell people it. Sometimes I wonder if its true or just an urban myth. Would love to know if there is a 'margin of error' built into the system.

CelticRambler

As you can only have one satellite directly over head and need three to triangulate your position, the other two should identify your vertical displacement.

But the day it starts to bother me whether some gizmo has fully accounted for a 5% gradient on my route that may save or cost me half a day per year is the day I'll book a one-way ticket on Virgin Galactic.