Speed, GPS -VS- Odometer

cormie

I have a function on my GPS (Airis T920a) that displays my current speed and although it can be inconsistent in that it will jump about 4 or 5kmph below the peak it has displayed even though I seem to be going the exact same speed (for example, a long, straight, flat road and I have my foot in the same position on the pedal and seem to be going the same speed, but it will display say 100kmph, then jump to 96, back to 99, back to 96 back to 100, but never above 100, once it has displays a steady peak, it wont jump to 104 for example).

Anyway, as confusing as that may have been to get what I'm saying, it always seems lower than my odometer. I was in a 120 zone and to get it up to 120 on the GPS meant it was up to between 130 and 140 on my odometer.

I've heard a million times that the Odometer displays about 10% higher than your actual speed and to add to that, speed cameras allow 10% on top of that too usually, so for me to get caught speeding in a 120 zone would require me to be going at least 150 on my odometer, which is fine with me as even on a long downhill straight, my van can't peak 140:D

I'm just wondering how accurate the GPS is

peasant

GPS would be pretty accurate (although with yours being so "jumpy" I wouldn't be so sure)

MY GPS kindly informed me, that while doing 100 on the speedo, my camper actually only does 88 ...which was confirmed by one of these "you are currently doing x speed" displays at the side of the road the other week.

When doing 100 according to GPS the speedo shows 118-ish.

On the Jimny I have bigger diameter tyres than original and the speedo is spot on ...no margin whatsoever.

Seth Brundle

Firstly (saying this for the second time on boards today), the gardai don't give 10%. They may give something but this is down to the garda doing the check.

Anyhow, could your GPS jumping be down to gradients? Does that affect speed figures with GPS?

peasant

I'd say the accuracy of your GPS would very much depends how many signals from different sattelites you're currently receiveing ...that and the quality of the software that does the extrapolation of all those signals.

The displayed speed on my GPS (Route 66 chicago) seems to be very finely tuned. I reacts to changes in speed faster than the speedo.
A slight blip on the pedal will hardly move the speedo needle, but the GPS immediatedly jumps up by one or two kilometers.

But my GPS is also deadly accurate (most of the time anyway) when it comes to location.

Turn-offs and crossrads are shown exactly where they actually are.

prospect

When my car reads 60mph (sorry i am not with the times yet!) the gps reads about 58mph.

P.

zuutroy

my speedo reads lower than the GPS above 80kmh. At 120 on GPS speedo reads 115, at 160 on the GPS speedo reads 148ish.

5500

Ive checked this recently with a garmin c510 and nuvi300 and both have matched the speedo exactly,i was also under the presumption of the speedo being slightly off too

Alun

eireal wrote:

Ive checked this recently with a garmin c510 and nuvi300 and both have matched the speedo exactly,i was also under the presumption of the speedo being slightly off too

What car is it? Do you have the standard manufacturer's wheels and tyres?

cormie

I have route 66 too but I have found sometimes it's a little late to react, I've taken the wrong turn off roundabouts before because it takes a few seconds to catch up to what exit I'm currently at, so I have to zoom out or just look at the exit number displayed.

The numbers you mention seem to be pretty much the same with me Peasant, 120ish when doing 100 etc, which is nice to know:)

I don't mean the garda giving 10%, I mean the cameras themselves;) plus then on top of that could be a friendly garda who'd give 10% again, sounds impossible to get caught speeding so hopefully my clean license will stay that way:)

I must say though, when you drive a lot, to different locations daily, a GPS is essential. Although some would disagree, I was at one place some day and a taxi man pulled up to drop someone off to the same place and he was shouting out "ahhhh ye don't have one of those GPS do you? Pen and ****in paper!!!" lol.

rebel.ranter

I would put the accuracy of GPS above the accuracy of a speedometer in a car. Most car amnufacturers try to ensure that their speedometers are at their most accurate between 50km/h & 120km/h as this is where monitoring your speed counts most.

However, bear in mind that the US government currently controls the GPS satellite network & can vary the accuracy whenever it feels like it. We use GPS for drive testing very regularly and whern you compare drvies from one month to the next you can see a marked difference sometimes.
I rememebr during the Iraq war there were a couple of work placement students in here doing a project on location based services, relying heavily on GPS. The day of their presentation and open day the GPS network was shut off so the the Iraqis couldn't use Guided missiles etc. Since then I have alwasy taken more notice of the GPS accuracy, I have added our office as a POI on the GPS software on my laptop, but while sitting on the balcony it can move according to the GPS!!! (I've heard of floating foundations but I think this is stretching it.)
Having said that using GPS for spped measurement doesn't really require the location to be accurate as speed is only a measure of relativity, i.e. how long it takes to get from one point to another, as long as the two points are out by the same amount the speed should remain reasonably accurate.
Interesting point about the gradients affect the speed reading too, I'd imagine that they woudl have to be reasonably steep to have a major effect though.

Cyrus

id go with the gps too they are very accurate i found by fitting larger wheels (in my case 18" to my e46 coupe as opposed to 17" it came with) the speedo actually matched the gps

cormie

what gradient are you guys talking about?

zuutroy

There's more to it than just how nice the yanks want to be. I find it hard to believe that they would just shut it off when so much air travel, sea travel etc relies on it. Vertical and horizontal dilution of precision and multi-point reflections limit the accuracy of the systems. Advanced GPS with fancy error correction can account to these factors better than cheap ones.

Seth Brundle

cormie wrote:

what gradient are you guys talking about?

I mentioned gradient as I was wondering if you were going downhill would it still see you as driving on a flat surface (i.e. its looking from above) and think you are going slower than you actually are.

Eoin

kbannon wrote:

I mentioned gradient as I was wondering if you were going downhill would it still see you as driving on a flat surface (i.e. its looking from above) and think you are going slower than you actually are.

I'll have to double check my GPS (nuvi 300), but I am fairly sure it can tell your height too, but whether it adds that into the speed equation is a different matter.

I've found that my speedo and GPS are always within 1km/h of each other.

zuutroy

They will use your absolute movement in all directions in calculating your speed, but even a steep gradient ia only going to cause an error of 5%.

gobby

AFAIK, speedos always read more than what you are actually doing. I can't back this up mind you. I heard it after talking to people at a car meet after noticing a difference between the speed as reported by the speedo and the GPS that was being used at the time.

Of course, there are plenty of other things that can affect this such as changing the diameter of your wheels. One of my friends dives a clubman with upgraded wheels. The speedo is not accurate at all! ( Might also have something to do with the b16a engine under the hood )

Seth Brundle

gobby wrote:

AFAIK, speedos always read more than what you are actually doing.

Seemingly speedos cannot underread - this does not mean that they will automatically overread!

gobby

Had a quick google on this (only a quick one, I'm in work ) and it seems to be the consensus that nearly all speedo's are up to +10% of your actual speed.

And, yes, by law they cannot under read

Seth Brundle

I believe that a lot of it is based on presumption. Many of those that checked their speedo against their sat nav found that their speedo was pretty accurate!

ctc_celtic

the error in speedo's will be more evident in older cars, espically cable driven speedo's.
newer cars wil be abit more accurate (electronic speedo), but still should read over, maybe only by 1 or 2 kph.

My 1995 MR2, with electronic speedo reads 114kph when doing 100kph.

gobby

The only time I personally witnessed this was in a mates smart car (four two). The difference was ~10% at ~70mph. That was a modern enough car. IIRC it was a '00 reg.

I guess it could easily have been a cable driven speedo. Also, we were using a laptop + USB GPS reciever. I can't remember the software though. It could also have been at fault!

ctc_celtic

what i meant to say was most cars will be about 10% out, but it is possible some mordern cars will be more accurate, but not all.
the car companys will never make a speepo completely accurate, because there are too many factors.
if the speedo was accurate and the tires were under (or over, cant remember) inflated then it would be possible to read under.
then if you got a speeding ticket, you could sue the car maker, as their equipment is faulty and lead you to brake the law.

rebel.ranter

zuutroy wrote:

I find it hard to believe that they would just shut it off when so much air travel, sea travel etc relies on it.QUOTE]

All I am saying is that on the day these guys were due to give their demonstration but couldn't due to the GPS system being down. We had 4 handheld GPS & 3 in-car systems with a group of guys very experienced in all things mapping/RF/IT/GPS. None of these systems worked this day. Anywhere. We moved location a number of times to see if it made a difference, it's the only time remember not being able to acquire on at least one GPS.

mkennedy

ctc_celtic wrote:

what i meant to say was most cars will be about 10% out, but it is possible some mordern cars will be more accurate, but not all.
the car companys will never make a speepo completely accurate, because there are too many factors.
if the speedo was accurate and the tires were under (or over, cant remember) inflated then it would be possible to read under.
then if you got a speeding ticket, you could sue the car maker, as their equipment is faulty and lead you to brake the law.

could they not just refute this by saying you should have had your tires properly inflated meaning you were ultimately responsibly

I know what you mean though
there are potentially other factors as you say which might be out of your control leading to such a scenario I suppose.

slickmcvic

the gps is very accurate,on Friday I got clocked by the cops (on a great stretch of wicklows N11!!!) knowing I was doing 120kph from my Satnav.
when i got pulled over the guards camera had read the exact same...i was execting some error but couldnt argue with that!!
My in car sppedo always reads higher than the GPS as well

PS no "You souldn't have been speeding in the first place" replies I'm sick of em on this forum!!!