How fast am I traveling | GPS Phones forum

jiananmail

GPS: TOMTOM XL250

How fast am I traveling

Hi everyone

When I drive, the speed displayed by TOMTOM XL250 is usually 5km slower than my car speedometer, so basically when my car speedometer shows I am driving at 80, TOMTOM only shows 75.

And it is the same for both of my two cars, so which one is more accurate?
My cars or the GPS: https://www.gps-data-team.com/convert/speed.php

Thanks

niol

GPS: XL 330 S

Re: Disagree

mikeman1485 wrote:

I think the car would know how fast its going since its calculating it without sending to satellites and all that mess

Here are two points one must consider when saying which is the most accurate in measuring actual speed.

1- Auto speeds are based on a certain tire, being at certain pressure, that results in a certain diameter of the tire. So low pressure, high pressure, or the use of tire that is not the exact same physical size of the tires that came on the auto can result in the speedometer being off either high or low.

2- While the information from the GPS satellites is exact, the speed you see on your display of your GPS is dependent on how often it samples your position in respect to the satellite position. Also, within your GPS is electronics that take the GPS data and converts it to speed to display. Depending on how accurate that formula is, will result in how accurate the speed displayed on the GPS is.

Summary- The best bet is to go out on a flat highway that has mile or kilo markers. Set your car speed to a speed you wish to check. Have a person observe, on a watch, the time in seconds. Travel for ten mile mile or kilo markers and note the time your friend said it took to cover the distance. Repeat again in case the markers are not exactly a mile or kilometer apart. Also note the speed your GPS indicates and what the dash is showing. Divide the time it took to cover ten miles or kilometers by ten. This gives you the time to cover one mile or kilometer. If the number is greater then one minute or sixty seconds your traveling slower then the speed you thought you where going. If the time is less then a minute then you are traveling faster then you believed you where going.

I own a TomTom XL 330 S and found it to agree with my speedometer dial and in time measured miles it was within one half of a second of being dead on. For me that is accurate enough.

I hope this helps some people out. Laughing

veyder

GPS: Navman

For me it was ~10kph

I've compared the readings of the TomTom with a few safety speed cameras and with my speedo - the speedo constantly showed 10 kph less than TomTom & cameras (they differed by 1-2 kph at most).

It wouldn't be a big deal as I could just recalculate the reading on the speedo but unfortunately my gf had to pass her drivers licence exam (she was passing it in NZ - you use your own car for the exam there). So not only she had to keep the speed under the real limit but also under the 50 kph as shown by the speedo. So in reality she had to keep it under 40 kph and make all other drivers really angry Very Happy

johmazz2006

GPS: Garmin Nuvi 765, Garmin 3950LMT, Garmin 2455LMT, Navman40, Garmin StreetPilot 3

speed

Go with the GPS indication rather than the vehicle speedo as thry are more accurate.
Car manufacturers are damn lazy with speed accuracy, and too ignorant to change the formula when putting different size tyres on the same vehicle.
Get the base car with high profile tyres and speedo is near to actual, but get the sport model with the low profile tyres and the speedo will read much higher than actual.

Guest

Bouncing markers / having a coffee at 2.5mph

- The most accurate speed reading is from Photo Enforcement cameras (redlight and speed cameras). They provide just under 100% correct reading. They are extremely simple to use: record your car speedo reading when you see the flash, and wait a few days to get a notice with your exact speed.
- The next best source is your friendly police officer with a radar speed gun. They happily assist to calibrate your speedometer, even if you do not ask.
- The next best source is your analogue speedometer. Stick to it.

- The last one is your trustworthy GPS using polished software tricks and hacks to minimise inherited flaws.

GPS satellites are positioned in a geostationary orbit, at least 36,000 kilometres (22,500 mi) above your car. Their margin of error can be up to 30 meters per a reading cycle. You can test how accurate is your GPS smartphone with this almost 20 years old GPS tool: https://www.gps-data-team.com/convert/speed.php
It is cool to use GPS speed as a guide, but do not rely on it (if caught speeding). Dedicated GPS units perform much better than any ordinary smartphone. You can check the reading error if you start the trip logger while sitting and sipping your morning coffee. The location marker will bounce non-stop. Do you still trust your GPS telling you how fast you are going?

duane4

GPS: Nexus 6P

Re: Bouncing markers / having a coffee at 2.5mph

Anonymous wrote:

- The most accurate speed reading is from Photo Enforcement cameras (redlight and speed cameras). They provide just under 100% correct reading.

Many have proven to be inaccurate too, people have beaten them in court.

Anonymous wrote:

GPS satellites are positioned in a geostationary orbit, at least 36,000 kilometres (22,500 mi) above your car.

This isn't correct, they can't be in a geostationary orbit and still cover the polar regions, 20,180 km (12,540 mi) according to wikipedia. IIRC they have an 11 hour orbit.

Anonymous wrote:

Their margin of error can be up to 30 meters per a reading cycle. You can test how accurate is your GPS smartphone with this almost 20 years old GPS tool:

They can be a lot more accurate and inaccurate then this, but for the most part they generally come within 3m accuracy, the faster you go the more accurate they get (to a point). Also as more birds become available, especially with multi-bands to get round atmospheric delays they're only going to get more and more accurate.

Anonymous wrote:

It is cool to use GPS speed as a guide, but do not rely on it (if caught speeding).

Having a log updated per second can get you off.

Anonymous wrote:

Dedicated GPS units perform much better than any ordinary smartphone.

What evidence do you have to back this up?

Anonymous wrote:

You can check the reading error if you start the trip logger while sitting and sipping your morning coffee. The location marker will bounce non-stop. Do you still trust your GPS telling you how fast you are going?

As above the faster you go the more accurate it becomes, walking speeds aren't that great.

Guest

Re: Bouncing markers / having a coffee at 2.5mph

duane4 wrote:

What evidence do you have to back this up?

To answer your last question first: My evidence level is ZZ (somewhere in between "I guess" and "I believe"). I trust that you, and anyone else, are much better GPS experts than I am.
BUT, the error can only increase as car goes faster. There is no logic that the GPS signal accuracy improves with speeding, BUT on the other side, as my friend explained to me, statistically, the confidence increases as the margin of error decreases.

Standing speed 0km/h - GPS shows 2.5 km/h - error = infinite
Walking speed 5km/h - GPS shows 2.5 km/h - error = 50%
Walking speed 5km/h - GPS shows 7.5 km/h - error = 50%
Driving speed 50km/h - GPS shows 47.5 km/h - error = 5%
Driving speed 50km/h - GPS shows 52.5 km/h - error = 5%
Driving speed 100km/h - GPS shows 97.5 km/h - error = 2.5%
Driving speed 100km/h - GPS shows 102.5 km/h - error = 2.5%

1. Any way, GPS does not show the exact speed! GPS software uses averaging to ESTIMATE your speed. Car speedometer uses exact MEASUREMENT to display your speed. In my opinion (evidence level: ZZ) car speedometer wins.

2. Smartphones share processor speed with any background running application. These apps delay internal clock reading and increase the time dependant calculation errors
Dedicated GPS units use a dedicated fraction of a processor time to get (more accurate) location reading and use that (more accurate) time for calcs.

I checked my speed with an old Apple iPhone, and a new Samsung smartphone. I am deeply sorry to say that Apple wins. Samsung's processor is too busy to give any good (time dependent) reading. (It does not mean that the iPhone is a better phone).

Anyway, I prefer my standalone GPS as it does not drain my battery as fast as Samsung/iPhone/any-other-smartphone.

duane4

GPS: Nexus 6P

Re: Bouncing markers / having a coffee at 2.5mph

Anonymous wrote:

1. Any way, GPS does not show the exact speed! GPS software uses averaging to ESTIMATE your speed. Car speedometer uses exact MEASUREMENT to display your speed. In my opinion (evidence level: ZZ) car speedometer wins.

The speed is based on the time between updates and the distance between updates, which is calculated not averaged, also many things can effect car speedo's, such as tyre wear, because they measure rpm off the engine, not ground speed.

Anonymous wrote:

2. Smartphones share processor speed with any background running application. These apps delay internal clock reading and increase the time dependent calculation errors

Smart phones like standalone GPS use a standalone GPS chip.

Anonymous wrote:

I checked my speed with an old Apple iPhone, and a new Samsung smartphone. I am deeply sorry to say that Apple wins. Samsung's processor is too busy to give any good (time dependent) reading. (It does not mean that the iPhone is a better phone).

That's hardly an exhaustive test of smart phones v GPS units.

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