Satnav Speedometer Accuracy

The car speedometer probably reads high. I know that my Miata speedometer reads a little high. One factor that can affect the car speedometer is tire size. I think they calibrate so that in no cases will the error result in the speedometer reading too low.

The police don't care what your speedometer reads. It's what the radar says that counts. The accuracy of GPS speed depends on a number of factors such as the quality of reception and number and locations of the satellite. Reception is bad under foliage, especially wet foliage. Accuracy is better if there are more satellites in view and they are scattered in different directions (you need at least 4 for any localization). GPS speed is calculated by making successive position measurements and dividing by time to come up with a speed, so a constant speed would be more accurate than a varying one.

The GPS receiver also determine accuracy. Not all receivers are the same.

Yes and no.

GPS may generally be accurate, but it is not guaranteed to be accurate.

From a legal point of view GPS is not accurate due to the potential for a number of factors to corrupt the calculated speed.

On the other hand, your speedometer very likely just reads fast. If there is an error, it is better to have it read fast.

You are only breaking the law if your vehicle exceeds the legal limit (plus any grace velocity - if your state or country has that - Florida has a 5 mph grace as per law).

Police use regularly calibrated tools to determine your actual speed, not your speedometer.

Your speedometer is probably wrong, but you can check that if you have a highway with regular mile markers and you travel at a known speed for several miles and accurately time the total distance traveled at constant speed. A passenger with a stopwatch works well. Convert the total time to fractions of an hour and then divide the total miles driven by the time. Note, if your speedometer is off, so will your odometer, so you need to use the highway mile markers.

My speedometer is also off, but I have just learned to compensate for it. In Florida (at least the areas I drive) I can routinely drive 10 mph over the limit (as measured by actual speed) and never get stopped even in the presence of police. I have done that for the 8 years I have been down here and nearly 100,000 miles. Never stopped once for driving that way. I drive a bright red sports car, too. Police are more concerned with the way you drive rather than how fast you drive.

There are some people that are simply speed Nazis. I have seen some with bumper stickers that state that their vehicle never violates the speed limit. I seen others get irate when someone passes them, flashing their lights, etc., when they do. I suspect that they are really no safer a driver than anyone else, but may be a bit deficient in their emotional IQ quotient.

The bottom line is to drive safely in a way that you are in control of your car and not a menace to others. Talk to police officers sometime and ask them what they are looking for when it comes to violations.

Mine's the same. I think factory speedos are all just a little fast to protect us from ourselves.

May be speedo is calibrated for the largest size of tire, keeping tire manufacturers specs in Mind,and tire of your vehicle are on lower side of spec,with some wear out of top layer. Just a Guess.

If you go around any curves, the GPS could compute a slower speed than the actual speed. It will also matter what the update interval is.

GPS, 3 smart phone apps all agree. My speedo is faster than they report.

You can get someone to measure your speed with a radar gun at say 20, 50 and 80 mph... this will show the curve of accuracy....You might just have lower profile tires installed...

"Dan Edmunds, an automotive engineer and the director of vehicle testing at Edmunds.com, says that speedometers cannot have an error of more than 5 percent (typically expressed as plus/minus 2.5 percent relative to the actual speed) according to federal law."

http://www.foxnews.com/leisure/2012/05/11/how-fast-are-really-going-accuracy-speedometers/

I've got a radar gun:

I'll test your speed if you live near Mesa, AZ.

$10.00 with official certificate.

An automotive engineer and director of vehicle testing ought to know better than that. There is no jurisdiction I know of which allows a speedometer to under-read. The regulation with the widest adoption is United Nations Economic Commission for Europe (UNECE) Regulation 39, in which the rules are (1) the indicated speed must never be less than the actual speed, (2) the indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. This implies a permissible error of more than +10%. In the US there are both Federal and local rules, but again under-reading is not allowed.
As for errors induced by tyre wear, pressure changes, etc they can be ignored. The difference in circumference between a new tyre and a fully worn tyre amounts to around 1%.

Remember to divide the satnav indication by the cosine of the gradient angle to obtain road speed.

Ermm, no. The satnav determines your positions in 3D space, so whether you are going up or on the level is irrelevant.

Car speedometers are generally optimistic. But 10% off is too much. Are wheels/tires the specified size? Because the satnav accuracy is potentially super super high, unless there's some serious issue in your specific device data acquisition software, that destroys data while trying to make some filtering/averaging. S.M.

Actually, the receiver can only do so much. Atmospherics, multi-path errors, and satellite positions can create more errors than even the best receiver can account for. In rare cases there may be an error in the actual SV satellite, but those are usually flagged pretty quickly and the satellite data labeled as defective.

For aircraft, which require a very high degree of 3D spacial accuracy, receivers usually implement WAAS (Wide Area Augmentation System) to help offset the effects listed above.

That may not likely be found in some of the lower end automotive receivers, but I think some of the better ones do have a form of it.

I've used GPS for a long time, way before it was so popular. I remember May 1, 2000 when they shut off the SA (Selective Availability) and accuracy went from 100 meters to more like 3-4 meters overnight!

Here is an interesting paper that describes GPS accuracy factors:

http://www.oc.nps.edu/oc2902w/gps/accfact.html

I remember that I couldn't use my first GPS receiver back on the farm in the woods because the canopy was too thick.

You can't see the house and barn on Google earth. Nor my brother's either.

I don't think phone apps have this problem.

Just downloaded a GPS app a few minutes ago. Now looking for an excuse to go somewhere.

It's been a few years, but my last aerospace job was GPS navigation for precision approach of aircraft.

Your accuracy is a little overstated, but what is considered statistically accurate for aircraft is more like "worst case" pseudorange accuracy of about 8 meters at a 95% confidence level.

The real problem is not lat/lon position accuracy, but vertical accuracy, which is not as good.

Surveyors use instruments that take a long term sample rate of days or weeks to get down to the sub-centimeter (if memory serves) using carrier-phase GPS.

My new GPS app tells me that it sees up to 15 satellites and is accurate to within 10 feet.

This varies, but......................................

I don't know if these figures are real, or just calculated or manufactured.

Just sayin'

Two comments:-

1) this was written before 2004. It really only applies to units built before 2000.

2) It even mentions changes that were due in 2005 to reduce errors to cm.

I feel that it is not really applicable anymore.....but still interesting, thanks.

Most towns will set up speed-check machines that tell you how fast you're going. Call your local PD and find out of they have one set up or an ajointing town does. Drive up to it and see what it reads, they are calibrated as police radar sets are, so it should read true. I would trust your speedo over satnav.

Luck is when oportunity and preperation come together. I have no idea who said it...

Welcome to the forum.

Check your tire air pressure.

Tire pressure would be a minimal issue and would most likely cause a LOWER speed by over-inflation. Under-inflation would cause a HIGHER speed reading but also cause unsafe handling issues. I run my tires about 4-5 psi over for better fuel mileage and better performance... a racing trick. If you chalk mark each of your tires at three locations on the outer edge for about an inch from where the tire meets the road (after you drive for about 15 minutes to get your tires up to driving temps), then drive around a bit... your driving habits will show how much you drive up onto your sidewalls CORNERING and indicate if you are driving with the proper tire pressures. If you wear out the chalk marks all the way up, you are driving with too low psi. If you wear out .25 inch of the chalk marks, too high psi, you should show about .4-.5 inch of chalk wear. This will help you get perfect tire wear according to how you drive and the road temperatures.

We have long sections of straight roads here in Australia. My Ford falcon 2002 speedo always shows 105 Km/Hr. at 100 KM/Hr. road speed. The GPS shows 100 Km/Hr. at 105 speedo reading. I trust the GPS reading since time, speed and distance always work out to 100 Km/Hr. road speed when the speedometer shows 105. Also a friend who was a Sargent in the police force followed me one day and told me I was doing 55 in the 60 zone, 75 in the 80 zone and 95 in the 100 zone. It was only then I realised why everyone else was passing me. Back in the 60s and 70s the police used to use the option of making you present a certificate of speedometer compliance if you were caught driving a bit over the speed limit. I think we call them the good old days. My advice is, as others here have said, is to do the maths. If you can drive 100 kilometres in one hour at constant speed then your speed is 100 Km/ Hr. regardless of what your speedometer is telling you. Use road side distance markers for the distance, not your odometer as also has been pointed out by others.

The Australian ADRs ( Aus Design Rules ) for new cars now state that a speedo must be accurate to +0 _ 10 K/h. It once was just "within 10%". As we rarely think for ourselves I believe our ADRs would be simply following UK and/or US equivalents.

Jim

A general thanks to everybody - and Lyn, but I doubt if I will ever be passing Mesa, Az for a speed check - it's 5,000 miles from here.

It seems there is some doubt about relying on the consistency of accuracy of the satnav for speed control. More reliable would be the car speedo if calibrated physically by time and distance readings.

The latter is difficult here nowadays because setting the car at a constant speed (quite tricky itself) even on a clear straight road, but made worse by driving safely in lots of busy traffic.

Either way, my basic problem is convincing my wife that we are not speeding, and staying below the speed limit according to the car speedo is the way we do it - whether she is driving - or in the back seat - bless her!

Thanks again.

Usually the law states in most countries that the Speedo may not show a lower speed than actual, but its OK to show a higher speed.

Years ago, the UK allowed a max difference of - 0% to + 10%.

Germany today has (if I remember correctly!) a max difference of - 0% to + 5%. This may even be a EEC ruling nowadays...I could not say.

Most Sat Navs I have used are accurate to +/- one unit. Miles or Kilometers, whichever is set, according to the manufacturer.

But accuracy is also affected by corners, so if you are on a very twisty road, accuracy then suffers, because the sat nav shows distance and speed "made good" (sailors will understand better!), not the actual speed relative to the road surface.

But if you are on a relatively straight road, Sat Navs are really good, better than any speedo, and are totally unaffected by tyre size, tyre pressure and tyre wear......

Once you know the difference, you can usually "correct" the Speedo reading in your head! Occasionally checking it again as tyre wears or are replaced with new ones.

I do this all the time and go through radar and other speed checking devices at the new and correct speed shown on the Sat Nav, or calculated in my head, no problems.....no tickets.

Most countries ignore minor infringements, so often you might have a further plus of say 5 MPH or similar!

Calculating approximate tyre wear for an 18" tyre gave me a less than 3% difference in circumference/speed from new to worn out, though I did not take all factors into account.....this will have been taken into account by the Speedo manufacturer.

Andy - I am inclined to go along with your explanation as it makes sense of all the parameters related to measuring speed in a car. The precision of the speedo or satnav is somewhat academic at times because the reading is displayed as a whole number - no decimal fractions, and beign digital - they jump a whole digit to the next reading.

As a slight diverence from topic - does the satnav calculate distance from satellite positioning only, or does it pick up distance from points on the internal road map. I ask because my satnav sometimes gets confused where I am when i take a 'wrong' turning, and it takes a while to realise I have taken a different road before it repositions me on the correct road on the map.

Just back from a spin.

The GPS app and the others all basically agree that my speedo is about 2 MPH fast.

Well within my tolerance.

Of all the Navs that I have used since 2004, none were "confused" if I took the wrong turning (just me!), but they allowed me to deviate only a short distance from the proscribed track, even going into a filling station would be marked as an error....and it would lead me back after a fill up.

Even my VW 2004 Navi from Blaupunkt did this very accurately.....that was my oldest unit....The CDs were far too expensive!!!

You needed your wits about you in a city where turnings off were sometimes very close to each other, you needed to check the display carefully. It was accurate, but the voice tips could be misunderstood I found....

Since then I only use Tom Toms......far better and portable!

They have one annoying error, the often pick the wrong car orientation when not moving, so I remember that and always drive 25 yards or so to see if I truly am pointing in the wrong direction or not after being caught several times having to change direction twice!!.....It happens on all the different Tom Toms I have owned....

Minor problem....the built in ones are better just in this respect, no others though!!! Portables are better otherwise....

My GPS has gone nuts.

On several occasions the GPS instructed me to turn the wrong way, giving me instructions to turn into a driveway of a housing development, follow a road around the inside of that development, back to the development entrance, then turn right past the point of initial error and onto the entrance of the highway without any apologies.

I have seen this behavior multiple times with my car's navigation system leading me on some circuitous route and then take me back the way I came to get onto the correct road like some drunk homing pigeon with Alzheimer's.

Sounds like you need a firmware and map update.....that should never happen.

Andy - again - thanks. I am probably confusing 'distance' with calculation time. ie, the TonTom knows exactly where I am, but I could travel quite a long way before it does the sums to re-calculate the route.

And AH- thanks - sorry for being flippant, but does the 'height' error with aircraft cause the plane to 'land' before it reaches the ground, or does it reach the ground before it 'lands'?.

I actually believe that a normal GPS unit is continually receiving Sat data, from (hopefully!) at least 3 Sats and is continuously calculating and updating the display.

Tom Toms do not have a "wheel" input, so when Sat info is lost in say a tunnel, it assumes that the speed remains the same, even if you stop in the tunnel!! Then it picks up the Sat signals again as soon as you leave the tunnel. Works well for me personally....

On my VW built in unit (with wheel sensors), it had several sensors so that even underground in say a parking place, the cars position was well reflected and orientation was also shown....a further plus of a built in unit, but not that important...

Vertical accuracy (for CAT I) is necessary primarily on the initial approach to establish a correct glide slope to the point of touchdown.

I believe WAAS GPS will get you down to 200 feet AGL, at which point local airport positioning takes over.

However, it is important to establish the correct glide slope early, so vertical precision is important, even if GPS is only used above 200 feet AGL.

I have no idea what year or model Honda, but many of the newer cars with gps have a calibration mode for the speedometer. You enter into one of the screens on the dash display and put it into calibrate mode and drive in an open area at as near a steady speed as possible and the computer calculates the error between the built in gps and speedometer and inserts a correction. I had to do this on my 08 Prius after changing tires and afterword it dead on. I have found out later that most new cars have this feature. If this is a fairly new Honda, you should check the owners manual and see if it has this feature. BTW the dealer didn't know about it until I told them. Typical "professional" mechanics.

I have driven Honda's for many years,and once the factory tires are replaced,the

speedo is 10% faster than actual speed.

I have verified this on long trips using mile markers on the interstates.

The odometer ,of course,also reads 10% high.

I can see where this would reduce the actual miles covered under factory warranty

by 10%,and increase apparent gas mileage by 10%.

Apparently,Honda uses a specially sized tire from the factory,and even though it is

replaced by a properly cross referenced replacement size,there is a difference in the

actual outside diameter of the tires.

I simply subtract 10% from my indicated speed and mileage to get actual numbers.

In a 70 MPH zone, I can safely do 80MPH(by my speedo) and not be over the allowable limit.

This small difference is not noticeable for short trips,but on 1000+ miles or more, it

can add up to several hours travel time.

I have not needed any warranty service so far in over 25 years of Honda's so the

warranty mileage issue is a moot point for me,however I think there have been some

legal challenges about warranty regarding this issue with Honda.

The speedometer for some late model Honda's are made by Ford,so I doubt that the

issue is with the indicator itself,rather it is in the pulse generator that sends the

signal to the odometer.(Imho).

I think my car reads 4 mph higher than the GPS, 55 -70 mph, interststate, straight-away. The tires are "slightly oversized" though. So, it probably makes sense.

Aside: I run Pirelli P400 tires and I have sinece 1982 on two successive vehicles. Actually they kept changing names (P4000, P4 and P400). Great tires. Dunlaps almost killed me hydroplaning. In my younger days without cheap tires, I would get 1/2" bolts in my tire, run over glass in the parking lot, have an ice pick or knife jabbed into the sidewall and slip into an eroded piece of road and 3 out of the 4 trim rings went rolling of the car and I bent the rim. In my early, early days like 16-17 years old, I flexed a rim around a corner that the hub cap fell off and was lost forever. I went around a corner with a safe speed of 30 mph at 50 mph.

I believe I am correct in saying that an oversize tyre will actually show a speed that is less than the actual speed, not what you said!!!

Also, the car will be considered illegal in most countries.....

I think, if the speedometer counts wheel revolutions as the mechanism to compute vehicle speed, you are right.

The circumference of a larger wheel covers more ground per revolution.

Speedometer. - Regulations Section | Federal Motor Carrier ...

Part 393

PARTS AND ACCESSORIES NECESSARY FOR SAFE OPERATION

< 392 | 395 >

• Section
• Guidance

§ 393.82: Speedometer.

The speedometer must be accurate to within plus or minus 8 km/hr (5 mph) at a speed of 80 km/hr (50 mph).

Citation: [70 FR 48054, Aug. 15, 2005]

On a practical note, here in Arizona most agencies will not stop and cite unless the vehicle is traveling at or above 11 MPH or over the posted speed limit.

I drive at 5 MPH over the posted limit on my way to work.

Our Director of Transportation told me that they set the speed limits on city streets at the speed where 85% of drivers are in compliance.

"Our Director of Transportation told me that they set the speed limits on city streets at the speed where 85% of drivers are in compliance."

If they used that rule in Chicago the speed limits would be 85 mph.