Speed check by satelite & vehicle

[Charlieme]

I find the satelite speed reading is always 2-3 mph slower than the vehicle reading speedometer.

 

The satelite screen positioned (in the front window) is easier to read than looking downwards to the small dial (behind the steering wheel)

 

Therefore when cruising at 40 within the law (by satelite) am I exeeding 40 at ground speed and bookable.

 

Would the satelite figure be upheld in law ?

[sshortcircuit]

I understand speedometers are set 10% higher.

 

Safer to go on the speedo than satnav, which is probably more accurate.

[Guest caraprof]

Manufacturers are allowed to set their speedos up to 10% higher, but not lower.

Consequently, most of them err on the safe side and set them higher. This also fools you into thinking that you're getting more mpg.

Your satnav is very accurate and if you stick to the speed limit as displayed by it you're not going to be prosecuted anyway, so the question of what the police will take into account doesn't arise.

It's actually very irritating being stuck behind some plonker who won't move over from the lane he's blocking because he thinks that he's keeping to 69 mph, when in fact he's doing about 63!

[Ralph]

I have used various satnavs for a number of years in a number of vehicles and they almost always come out at about 5 mph slower than the speedo.

When I was "snapped" by the constabulary their figure tallied with my speedo (Honda CRV at the time).

 

I have read various threads before on this subject and basically you can't rely on either. Frank is right that speedos are often (but not always) set high but satnav is far from accurate. One reason is that they don't take gradient into account and always assume you are on the flat. There's lots more technical stuff but I'm not qualified to understand it, still less spout it :-D

 

Try http://autos.groups.yahoo.com/group/motorhome-list/message/71965 for starters

 

 

[Miami]

Well you learn something everyday!

I always thought my GPS speed was the accurate one. Good job I haven't got caught so will ignore it in future.

[Ralph]

Sorry, I hadn't realised that my link wouldn't work unless you are a Yahoo groups member.

So here's the stuff I was pointing to. If you want to read the rest of the postings you'll have to join the Motorhome List:

 

 

 

Many have commented about the use of GPSr speed displays.

 

||> I have tried several different GPS's on several different

||> cars and vans.

||> In every case I got a reading on the GPS of around 5 MPH

||> lower than the

||> speedo. So either all those vehicles read 5 MPH high or GPS speeds

||> aren't accurate.

 

The speedo measures the number of road wheel rotations converted to

display mph. New tyres on high pressures have a larger circumference

than when worn and on low pressures. Different tyre makes and sizes

have a different circumference.

 

I've just changed my motorhomes tyres from Michelin XZL 7.50 R 16 C to

BFG LT 235 85 R16. (B F Goodrich is a Michelin brand.) I have been in

conversation with Michelin about this and it's their recommendation,

incidentally.

 

The circumference difference is small.

 

Speedos have to cope with these changes, so are not very accurate.

 

I have also compared speedos with my GPSrs and found similar results.

 

Opinion of GPS experienced users (who have knowledge of how they work)

is that indicated GPS speeds are accurate to 1 mph. But as the next

poster wrote:-

 

||> They do not adjust for hills so if you are hurtling down a

||> 1 in 4 hill at a GPS speed of 55 mph, your true speed will

||> be nearer 60, your speedo will register 60-66, and you'll

||> never make it around the corner at the bottom :-)

 

This is also true, subject to the math being correct. A GPS speed is

the horizontal speed.

 

But beware that the software displayed speed is not always the GPS

calculated speed. The software while busy moving the map display,

plotting the changed location plot, calculating the distance to the next

turn, etc.; will still be displaying an old speed, so you could be going

faster or slower.

 

Also when slowing for a speed camera, the GPS speed will be an average

speed over one second!

 

Also when a GPS fix, or fixes is/are lost the average will be over a

longer period, so not that accurate.

 

GPS fixes are lost for example when the Luton top reduces the radio

signal received. While folk will say that their GPS works OK under

their Luton top, remember that it will still give a location plot on the

screen, but is less accurate.

 

Newer GPSrs with navigation fool the user into thinking that their GPSr

is more accurate than it actually is. How? Road 'lock' converts (=

reduces accuracy) the GPS reported location, so that you remain on a

road. Also when fixes are lost, the navigation software can display a

location that it calculates, based on the last reported speed - but you

may not actually be there!

 

Watch the pointer flip at a junction, often in a town, when you take a

wrong turn. The pointer continues on the 'correct' road and then flips

as it gets a new fix, along the wrong road that you took.

 

The best way to use the GPS speed is to drive on a level road in open

countryside at a constant speed, say 40 mph and note the speedo reading.

Then always use the speedo for you real speed.

 

Alan Morris.

 

[GJH]

caraprof - 2007-10-05 11:43 PM

Manufacturers are allowed to set their speedos up to 10% higher, but not lower.

Consequently, most of them err on the safe side and set them higher. This also fools you into thinking that you're getting more mpg.

Your satnav is very accurate and if you stick to the speed limit as displayed by it you're not going to be prosecuted anyway, so the question of what the police will take into account doesn't arise.

It's actually very irritating being stuck behind some plonker who won't move over from the lane he's blocking because he thinks that he's keeping to 69 mph, when in fact he's doing about 63!

Let's look at this logically for a moment. Will the driver behind the plonker really be irritated?If most speedos are set high then the one in the vehicle behind the plonker is probably also reading closer to 69 mph rather than 63 mph. So, the driver behind, being a law abiding person, will assume that he/she is close to the speed limit so will really be thanking the plonker for preventing him/her from inadvertently breaking the limit and risking being caught for an offence.Of course, the driver behind might have taken the trouble to have the speedo on his/her car accurately calibrated so will know for certain that the true speed is 63 mph. In that case the driver behind will really be thanking the plonker for saving him/her the expense of having to have cruise control fitted to achieve fuel economy.Wonderful thing, logic (lol) Graham

[Guest caraprof]

Not much logic in that response I'm afraid. The truth is that there is a very good chance that the driver being held up has a satnav and is aware that the true speed is possibly seven miles an hour lower than the allowed one. But the point that I was making if you read my post properly is that it is drivers who won't move over that are the problem

These smug, self satisfied idiots are too thick to know that speedos can often be set high and sit in the outside lane saying to themselves: "I'm doing 69 mph so why should I move over?"

This is frustrating to begin with but when you know that he is going nowhere near the limit it becomes even more so.

But of course, logic is a very silly thing to apply as the first rule of driving behaviour. Someone who wilfully blocks a lane by refusing (illegally) to obey the 'Keep Left' rule can actually be the indirect cause of many accidents and deaths because regrettably, some people are impatient and when they become incredibly frustrated by what they see as unreasonable behaviour, they take silly risks.

We've all seen the idiots who tailgate and overtake on the inside. What they do isn't right, but the fact that they do it is inescapable and anyone, who contributes to driver frustration by wilfully refusing to move over because he's convinced that he's driving at the limit, is an irresponsible and dangerous fool.

[Guest caraprof]

Miami - 2007-10-06 9:39 AM

Well you learn something everyday!

I always thought my GPS speed was the accurate one. Good job I haven't got caught so will ignore it in future.

Despite what has been said, your GPS speed will always be more accurate than your speedo. Whilst it's true that GPS speed readings can be very slightly out on hills the point made by another poster is sensible in that it's easy to work out how much your speedo is out by comparing it to your satnav on a flat road.

I know for instance that my speedo reads about 4 mph under at 70 mph and that at 40 mph it's between 2 and 3 mph under. I am very happy driving my car when conditions allow with the speedo reading lightly high to allow for this.

And of course you can lose your signal, but so what! If you lose it you go off the speedo and add 2 mph or whatever you know is accurate!

Apart from which I don't believe that anyone has ever been convicted of speeding for driving at 32 mph in a 30 area and 43 mph in a 40 mph area, assuming of course that conditions were perfectly safe to do so. I actually shouldn't have to say this as it should be obvious, but I know from experience that on this forum, someone will leap right back with a statement of the bleeding obvious and say that driving at 32mph through a crowded town centre is dangerous and will result in a conviction. Yes, we know that thank you!

[Clive]

Believe the GPS derrived speed.

C.

 

[GJH]

caraprof - 2007-10-06 11:12 AM

Not much logic in that response I'm afraid. The truth is that there is a very good chance that the driver being held up has a satnav and is aware that the true speed is possibly seven miles an hour lower than the allowed one. But the point that I was making if you read my post properly is that it is drivers who won't move over that are the problem

Ey up Frank, didn't you see the smiley?You need to exercise your chuckle muscle old lad (lol) Graham

[Charlieme]

Good replies.

 

Going slightly off track

 

(1) Is that why the truckers with Ireland plates always go faster.

 

(2) If I fitted low profile tyres (assuming) then would that change the manufactures speedo gearing?

[Guest starspirit]

The GPS speed lags a second or two behind the speedo speed which can be a bit of a bu##er when passing a camera.

[Ralph]

Charlie&me - 2007-10-06 7:08 PM

 

(1) Is that why the truckers with Ireland plates always go faster.

 

(2) If I fitted low profile tyres (assuming) then would that change the manufactures speedo gearing?

 

(1) No, it's because they changed over to KPH but the drivers still use MPH speedos :-D

 

(2) Yes, I believe that's so.

 

 

[Guest starspirit]

Lower profile tyres will revolve more times per mile than the originals thus making your van a bit slower at the same revs and your speedo over read even more.

 

If you go to www.alloywheels.com there is a calculator that shows you the effect on speedo readings of different size wheels and tyres.

[davenewellhome]

GPS is far more accurate than your vehicle's speedo but it will usually be 1 to 2 seconds behind, not a real problem for sensible drivers. GPS does NOT get confused on speed measurement by going up or down hills as it is a three dimensional locating system. GPS units typically take a location every second and then by simply calculating distance travelled during that second since the last location can display a near to real time speed but of course its at least one second behind.

 

Use your speedo as it will give you a safe margin for avoiding speeding tickets.

 

D.

[Clive]

If you fit low profile tyres you should use bigger rims thus keeping the rolling radius the same. Fitting low profile tyres on existing standard rims is just a bodge.

 

 

[GJH]

davenewell@home - 2007-10-06 9:10 PM

 

Use your speedo as it will give you a safe margin for avoiding speeding tickets.

 

D.

Yes. Pity more people don't follow that suggestion.

 

Graham

[Guest caraprof]

I can't argue with that if all you have is a speedo. But, if you know from constant use of your GPS that your speedo is say, 6 mpg slow at 70 mph then driving at 64 mph when the conditions safely allow a higher, still legal speed, is selfish.

It's a sad fact that many of those who toddle along at much less than what would be a safe and legal speed also tend to be members of the Middle Lane Owners' Club and reduce what could be a free-flowing motorway to a bunched up nightmare.

50% of the 3000 road deaths each year occur after one driver has perceived a discourtesy by another. Tempers flare and risks are taken with the inevitable results.

Finally, please don't anyone complain that the risk takers have only themselves to blame. We all know that but regrettably, human nature is human nature and there are some silly short-tempered people out there and anyone deliberately or thoughtlessly holding up drivers who have a legal right to go that little bit faster must bear a lot of responsibilty for what may be unpleasant consequences.

[Mel E]

Dave,

 

A minor point, but though GPS can obtain an altitude reading, the accuracy is quite poor compared with the accuracy of the positional data. I would be surprised if ordinary automotive GPS devices used the altitude data, but I'm prepared to be surprised.

 

Even if they did, at 60 mph, you travel 90 feet in a second, quite enough to descend into a dip and ascend out again.

 

I always assume that GPS is giving an accurate speed reading ONLY on the level. It's interesting that, in the many-miles-long roadworks section of the M1 north of Hemel Hempstead that was restricted to 40 mph and is now at 50 mph, I always set my cruise control to the GPS limit and stay in perfect position in relation to trucks and buses - suggests they're using the same data input!

 

Mel E

====

[GJH]

caraprof - 2007-10-07 4:09 PM

I can't argue with that if all you have is a speedo. But, if you know from constant use of your GPS that your speedo is say, 6 mpg slow at 70 mph then driving at 64 mph when the conditions safely allow a higher, still legal speed, is selfish.

Perhaps the important word here is know. Does any particular driver know that his GPS is more accurate than his speedo or only believe it to be the case? I suspect that few drivers have actually had both GPS and speedo scientifically calibrated.There have been sufficient posts on this thread to suggest that the accuracy of GPS isn't necessarily as good as one might like to believe.Graham

[Mel E]

IRISH TRUCKS

 

Their tendency to exceed the speed limit on motorways really has nothing to do with mph or kph. All trucks above 7.5 tonnes MAM throughout the EU have, for several years been governed to 90 kph/56 mph.

 

I assume that some Irish drivers have discovered a way of tampering with these governers.

 

Mel E

====

[davenewellhome]

Prepare to be surprised Mel, GPS is a totally three dimensional positioning system and they will all take a three dimensional position. The more expensive systems will often have the ability to display height above sea level while the cheaper ones don't but that doesn't mean they don't work in three dimensions. As long as they are receiving more than three satellite signals they will always work in 3D. You are correct about the distance travelled at speed and therefore a small inaccuracy occurs when descending or ascending short hills or taking corners. however the GPS is far more accurate than the vehicle speedo overall.

 

D.

[bigal55]

davenewell@home - 2007-10-07 7:01 PM

 

Prepare to be surprised Mel, GPS is a totally three dimensional positioning system and they will all take a three dimensional position. The more expensive systems will often have the ability to display height above sea level while the cheaper ones don't but that doesn't mean they don't work in three dimensions. As long as they are receiving more than three satellite signals they will always work in 3D. You are correct about the distance travelled at speed and therefore a small inaccuracy occurs when descending or ascending short hills or taking corners. however the GPS is far more accurate than the vehicle speedo overall.

 

D.

i agree, i been diving for 20yrs, using gps for 10, the more you pay the better the accuracy, the one we have now at worst puts us 10mtrs away from our target

[KD]

on a road near me one of those camaras that tells you your speed doesnt nick you but tells you to slow down if going too fast or thanks if you are ok always on all my 4 vehicles gives a slower reading than my speedo roughly 10%