Hey Everyone....

I like to keep my Navman on all the time while im driving (I hard-wired the charger into the back of the dash :D) and often watch the speed indicator on it, but it seems to always report that I'm travelling 3-5km/h slower than my speedo indicates.

This morning I downloaded a speed application for my iPhone GPS and it says the same thing..... about 3-5km/h slower than the speedo.

I have tested this at 100km/hr and 60km/hr.

I'm running original stock verada rims and standard tyres fitted by a tyre place. No drivetrain mods at all.

Is the speedo out????? What do you think guys...

i would not go on the gps estimate

it is getting readings at intervals, it's not continuous.

also your trusting readongs from satellites kilometers in the sky etc.

your speedo may be off by a few km/hr due to tyre size, tread etc. but will still be more accurate and reliable than the gps

But the two GPS's (Navman and iPhone) are saying the exact same reading..... I have tested it at two speeds

Hey Everyone....

I like to keep my Navman on all the time while im driving (I hard-wired the charger into the back of the dash :D) and often watch the speed indicator on it, but it seems to always report that I'm travelling 3-5km/h slower than my speedo indicates.

This morning I downloaded a speed application for my iPhone GPS and it says the same thing..... about 3-5km/h slower than the speedo.

I have tested this at 100km/hr and 60km/hr.

I'm running original stock verada rims and standard tyres fitted by a tyre place. No drivetrain mods at all.

Is the speedo out????? What do you think guys...

There are three things to consider here that i can see

1. Your speedo was manufactured by Mitsubishi for the exact specifications of your car

2. The signal from your GPS has to travel out to space and back again to calculate you speed.

3. If you beleive your GPS and you get pinched for speeding "Sorry officer is was doing the speed on the GPS" isnt going to cut it.

That being said, I had the same problem in the Road Train i took out west and because we had deadlines to meet we used it to justify going a little faster lol

Kieran

look in motor mags and they do actuall testin @100kph... all cars vary from -5kph but never over.

My gps does the same.. about 5kph out @ 100kph

Yeah...... I'm not looking to start a fight or anything and I don't disagree with you guys completely, i just thought it was interesting. I would presume that GPS's are calibrated to compensate for moving at those speeds but I would of course trust my speedo before my GPS.

Does anyone reading this have any experience with this kind of technology, your input would be appreciated...

If you are continuously changing speed, don't trust it, but when you are sitting on a freeway with the cruise on, they would be pretty bloodie accurate... more so than your speedo.

If you are continuously changing speed, don't trust it, but when you are sitting on a freeway with the cruise on, they would be pretty bloodie accurate... more so than your speedo.

What makes you say that?

I tested them on a straight stretch of road with cruise on.... Came to about 3km/h off at best

What makes you say that?

When you are sitting at the one speed on a flat straight piece of road, like a freeway, you take out the margin of error it will have when your accelerating, slowing, sitting in traffic, etc.

I remember reading an article on it a while back where it was determined that in freeway driving, GPS units are accurate to within about .2 of a kilometre.

also , the few times that I have passed those signs that display your speed on freeways why the GPS is on, the times always match up perfectly.

I have 3 GPS's and 4 instrument panels for the TR.

I did a HEAP of tests a while back over a measured distance using all the dashes and GPS's because of just this question.

I found that the GPS was the least reliable in measuring speed.

They do not take into account the slope of the ground and corners. They make calculations based on intermittant fixes from the sat's.

So if you are going around a long corner the gps works on straight lines between each fix and give an erronious reading. The same applies for hills as the sat fixes are 2 dimentional and the ground is 3D. Effectively the gps measures your time to cross the bottom of a triangle (cutting the hill through in a side view) while the car goes along the longer hypotenuse of the same triangle.

I found 3 of the 4 dashes read the same as each other (1 had no idea) and were much closer to correct when speed was calculated using time versus distance.

The upshot is .................... the gps's may be close to each other (as they all use the same method to calculated speed) but they are not that accurate (they are closer on dead straight, flat roads, at a constant speed but how often does that really happen, even on a freeway).



And no ........ I dont get out much lol :D

I have 3 GPS's and 4 instrument panels for the TR.

I did a HEAP of tests a while back over a measured distance using all the dashes and GPS's because of just this question.

I found that the GPS was the least reliable in measuring speed.

They do not take into account the slope of the ground and corners. They make calculations based on intermittant fixes from the sat's.

So if you are going around a long corner the gps works on straight lines between each fix and give an erronious reading. The same applies for hills as the sat fixes are 2 dimentional and the ground is 3D. Effectively the gps measures your time to cross the bottom of a triangle (cutting the hill through in a side view) while the car goes along the longer hypotenuse of the same triangle.

I found 3 of the 4 dashes read the same as each other (1 had no idea) and were much closer to correct when speed was calculated using time versus distance.

The upshot is .................... the gps's may be close to each other (as they all use the same method to calculated speed) but they are not that accurate (they are closer on dead straight, flat roads, at a constant speed but how often does that really happen, even on a freeway).



And no ........ I dont get out much lol :D


Thanks..... that pretty much answers my question!!!!

The thing to bear in mind is your speedo will always read higher than what your actually doing, they come out of the factory like that, set to read 2-3% higher than actual speed.

The GPS will be accurate on flats in a straight line at a constant speed like said, so it'll give you a rough indication of how far out the speedo is, but I wouldn't go using it 24/7 to check speed.

So, if you want an idea of what your speedo should really be reading, either get it re-calibrated to exactly what your doing, or take the car out on a flat straight road, set the cruise to an easy speed to fix to on the cluster (ie. 60, 80, 100 etc) then continue along that path until the GPS is holding a constant pace as well. The GPS will be measuring how quickly you travel between 2 points, and if held constant, should give a constant reading.

That should tell you accurately how much your speed is out, and you can work that out in your head as you drive if you want to stay as spot on the speed limit as possible.

The variation should work out as a percentage unless your speedo is stuffed and never drops to 0 or drops below 0 when stationary. ie. if its 5% out, when your actual speed is 60, displayed speed will be 63 or 57, actual 100, displayed 105 or 95 etc etc. Whether displayed is under or over actual will depend on which way it is out.

Yeah about 3km off at 100km/h on a straight road, That is what I was thinking in my first post, that the speedo is set to be 3-5km out from factory

How far out was it at 60? around 62? Should be 1.8km/h out to be specific if its the calibration.

How far out was it at 60? around 62? Should be 1.8km/h out to be specific if its the calibration.

It wasnt much..... I did the 60 test a while ago and thought I might get a greater difference with the 100 one......

I might try again and report back

I've been using http://www.tempes.com/ on my phone and have found it relatively accurate compared to my speedo.

my N95 reports almost the same speed as my dash

my N95 reports almost the same speed as my dash

my n95-i's (the black ones, and yes i have two :S) both indicate the same speed vs dash speedo.

when my dash tells me 100, gps tells me 102...

its normal for car manufactures to pump up the speedos a fraction, so that it saves them from being sued by realeasing a car that can get people booked for speeding basically..

(so for the above.. im a lil confused as to why the factory varience seems to have gone the other way for my car!! have to test it with a *real* gps tho, so that might explain it..)

my2c

My GPS is usually 3km/h below my speedo. Which is good, I know I can safely do (say) 103-104kmh on the speedo and still not get snapped.

Regards, tony

Oh dear. A lot of misinformation in this thread. Once cruising GPS speed calculations are as accurate as civilian uses will ever require. GPS does account for altitude, and therefore altitude changes - altitude can be calculated as long as there's 4 satellites to track (and most GPS units should easily be able to hold onto 6 or more). As for acceleration, I don't notice much lag with my N95 following speed change at all (less than a second), although it doesn't display decimal speed once into double digits I think. As for corners, well, it's a corner, perhaps watch what you're doing?

my N95 reports almost the same speed as my dashMine was reading 62 at indicated 65.

As a point of reference, a mate of mine with a RenaultSport Clio has a Garmin GPS. The speed it displays is 5% lower than the indicated speed, but the car is also equipped with a computer which mines data from everything, including the ABS speed sensors, and the ABS is dead in line with the GPS calculated speed.

I use my Navman for speedo calibration, as well as for time calibration.
As the Navman refreshes after several seconds, it can't be used for instant speed. However it is good for a short-term average speed. To check your speedo error, find a straight stretch and hold each speed for up to 30 seconds each. After setting my tyre pressures (40psi front / 34 psi rear / 45 psi spare), I tested my speedo, and found it to have a consistent 5% over-read (Speedo 105 Km/h = real 100Km/h). The accuracy of the Navman's speed calculations has proven its worth as I pass numerous speed cameras at +5% speed on the dial. Here in Victoria they set a speed tolerance of between 2 to3 Km/h only (They'd rather we watch the speedo than the road).

Has anyone purchased updated maps for Navman?

They do not take into account the slope of the ground and corners. They make calculations based on intermittant fixes from the sat's.

So if you are going around a long corner the gps works on straight lines between each fix and give an erronious reading. The same applies for hills as the sat fixes are 2 dimentional and the ground is 3D. Effectively the gps measures your time to cross the bottom of a triangle (cutting the hill through in a side view) while the car goes along the longer hypotenuse of the same triangle.

I

I Agree with Ol'Fart on this


I never trust my Navman for speed readings as it never says the same as my speedo

The accuracy of the Navman's speed calculations has proven its worth as I pass numerous speed cameras at +5% speed on the dial. Here in Victoria they set a speed tolerance of between 2 to3 Km/h only (They'd rather we watch the speedo than the road).

Why tempt fate?

3. If you beleive your GPS and you get pinched for speeding "Sorry officer is was doing the speed on the GPS" isnt going to cut it.
Kieran

It has been proven before that a GPS was used to contest a traffic ticket with success. But only in America of course. However a good GPS is usually accurate to about 2kms/h. It depends on how many concurrent satellites its locked on to.

http://techdirt.com/articles/20080718/1234331725.shtml

The signal from your GPS has to travel out to space and back again to calculate you speed.



For your interest, as I think you may be interested in such things, there is no signal sent from the GPS receiver to the satellite.

The way the GPS receiver calculates its position is quite clever. There are a network of satellites (I think about 30 or so) used for GPS which have synchronized clocks. They emit a time signal and satellite ID signal several times a second. Depending on how far you are from each satellite, the GPS receiver will receive a different time signal from each of the satellites. It can then calculate the distance it is from each satellite (it would receive a later time eg 7:05.05 am from a satellite closer to it than one further away 7:05.01 am).

If it can see three satellites it can triangulate a position on the surface of the earth. If it can see four satellites it can also tell the altitude.

Oh dear. A lot of misinformation in this thread. Once cruising GPS speed calculations are as accurate as civilian uses will ever require. GPS does account for altitude, and therefore altitude changes - altitude can be calculated as long as there's 4 satellites to track (and most GPS units should easily be able to hold onto 6 or more).

Like I said, I have 3 using 2 different software suites.

None have ever matched reality when it comes to height, especially when you pass a roadside sign on a range and the gps is out by a mile. I have even had 1 reading way below sea level when driving on a road by the beach.

Its just not something the gps designers seem particularly worried about.

What models were they?

I think we would agree that the "car" GPS device was never intended to be used as a substitute for anything other than a street navigation device. The speed (kms/h) are only indicators and such are only the selling points "bells and whistles" that each manufacture puts in to make them attractive to the mass market. I.e. such as mp3, blue tooth, camera built ins etc...

On the Tom Tom site that have stated a disclaimer...
"TomTom is not liable for errors in the maps and accepts no responsibility if you are caught breaking the speed limit."

I find the speed on the GPS is always DIFFERENT to the speedo, but the difference is CONSISTENT. The GPS is more ACCURATE. In most cars you can expect the speedo to show 105kmh and hour at an indicated 100kmh. It will usually range between 100-110. As long as your speed is consistent and you are travelling straight and the GPS has a satellite lock it will be very accurate.

Ollie

The unwritten rule of sat nab systems is that they are 4 km/h behind.

It has been proven before that a GPS was used to contest a traffic ticket with success. But only in America of course. However a good GPS is usually accurate to about 2kms/h. It depends on how many concurrent satellites its locked on to.

http://techdirt.com/articles/20080718/1234331725.shtml

In Australia as well actually.

In most cars you can expect the speedo to show 105kmh and hour at an indicated 100kmh.
I would expect it to show 100km/h at an indicated 100km/h, otherwise my eyes would be playing tricks badly lol

Actually - GPS units are more accurate.

How do I know this? Well I am a senior hardware engineer for Topcon Positioning Systems (whom utilize GPS, our own designed receiver the G3, to track asphalt and concrete pavers all across the world)..

As previously stated, with more than 4 satellites locked a GPS unit (Sirf III, which most GPS units use) will take into account any errors introduced by changes in altitude.. Also, cornering will not introduce any speed errors...

/quote
On the Tom Tom site that have stated a disclaimer...
"TomTom is not liable for errors in the maps and accepts no responsibility if you are caught breaking the speed limit."
quote/

This is only because the maps have speed limits and speed camera locations in them which quite often are not correct.. Not because they display actual speed.

However, if I were driving through a tunnel where satellite reception dropped out completely and the GPS unit used dead reckoning to establish position, then I would expect speed to be incorrect.

My GPS with I-GO software reads about 5kph less than my car speedo and the speed limit display changes almost instantly the time I enter different speed zones. I have gone through a few speed cameras at the speed limit indicated on the GPS and been fine so I think it's pretty accurate. I don't know about anyone else but I have a habit of only looking at the GPS speed readout as opposed to car speedo now. Oh one more thing, I believe GPS does take into consideration altitiude because I get an elevation reading which when driving over the mooney mooney bridge says about 75-80 metres which is roughly how high the bridge is above the water.

I would expect it to show 100km/h at an indicated 100km/h, otherwise my eyes would be playing tricks badly lol

Ooops, I meant indicated 100kmh at actual 105kmh, but im sure you already knew that lol

Ollie

my speedo and Navman match as close as i can tell when on constant speed. When i was running non standard tyre size it was out by the same amount calculated from the tyre size. I am confident the gps is acurate