Does a Garmin give you a true speed reading?

Given that a GPS has to be incredibly accurate measuring time (to work out where the satelites are) and will usually be accurate to a few metres, the speed measurement must be very accurate if calculated over a reasonable distance (eg 1 km or so when errors in the distance measurement will be very small as a percentage of the distance travelled. So must be much more accurate that than the speedo.

I think also that speedo is based on wheel revolutions? Therefore as tyres wear speed measured will differ.

No!The GPS system has a built in accuracy rate that can, and is changed at times.so the speedo gives a far more accurate reading.

As for the above posting huh. " GPS is usually accurate to a few metres but with tyre wear the speedo will differ " ? The tyre difference due to wear is in the MILLIMETRES NOT METRES!!!!!!!!!!!!!

Some people just don't get it!!!!!!!!!!!!!!!

Given that a GPS has to be incredibly accurate measuring time (to work out where the satelites are) and will usually be accurate to a few metres, the speed measurement must be very accurate if calculated over a reasonable distance (eg 1 km or so when errors in the distance measurement will be very small as a percentage of the distance travelled. So must be much more accurate that than the speedo.

I think also that speedo is based on wheel revolutions? Therefore as tyres wear speed measured will differ.

HOW ACCURATE IS THE SPEED READING?

If you are completely stopped, Selective Availability (the "noise" put into the GPS by intent to prevent military use by unfriendly folks" --See: http://joe.mehaffey.com/sabysam.htm ) causes appearent motion despite your GPS being at a fixed location. Typically this speed is in the range of zero to about 3mph (5kph)

Take a look at the Garmin site, www.garmin.com - About GPS link for a good explanation of the technology. The bottom line is that typical civilian GPS units are accurate to within 60-225 feet, according to Garmin.

At 120 mph you are traveling 10,560 ft/min. 60 feet more or less is a difference of .0057 (10,500/10,580), or + or minus 0.7 mph. If the error was 225 feet, the speed would be off by + or - 2.5 mph. The figures work out about the same at 60 mph or 180 mph.

I would not class 60-225 ft incredibly accurate in any way shape or form

Given that a GPS has to be incredibly accurate measuring time (to work out where the satelites are) and will usually be accurate to a few metres, the speed measurement must be very accurate if calculated over a reasonable distance (eg 1 km or so when errors in the distance measurement will be very small as a percentage of the distance travelled. So must be much more accurate that than the speedo.

I think also that speedo is based on wheel revolutions? Therefore as tyres wear speed measured will differ.

HOW ACCURATE IS THE SPEED READING?

If you are completely stopped, Selective Availability (the "noise" put into the GPS by intent to prevent military use by unfriendly folks" --See: http://joe.mehaffey.com/sabysam.htm ) causes appearent motion despite your GPS being at a fixed location. Typically this speed is in the range of zero to about 3mph (5kph)

Take a look at the Garmin site, www.garmin.com - About GPS link for a good explanation of the technology. The bottom line is that typical civilian GPS units are accurate to within 60-225 feet, according to Garmin.

At 120 mph you are traveling 10,560 ft/min. 60 feet more or less is a difference of .0057 (10,500/10,580), or + or minus 0.7 mph. If the error was 225 feet, the speed would be off by + or - 2.5 mph. The figures work out about the same at 60 mph or 180 mph.

I would not class 60-225 ft incredibly accurate in any way shape or form

Thank you, I know have my explanation.

Given that a GPS has to be incredibly accurate measuring time (to work out where the satelites are) and will usually be accurate to a few metres, the speed measurement must be very accurate if calculated over a reasonable distance (eg 1 km or so when errors in the distance measurement will be very small as a percentage of the distance travelled. So must be much more accurate that than the speedo.

I think also that speedo is based on wheel revolutions? Therefore as tyres wear speed measured will differ.

HOW ACCURATE IS THE SPEED READING?

If you are completely stopped, Selective Availability (the "noise" put into the GPS by intent to prevent military use by unfriendly folks" --See: http://joe.mehaffey.com/sabysam.htm ) causes appearent motion despite your GPS being at a fixed location. Typically this speed is in the range of zero to about 3mph (5kph)

Take a look at the Garmin site, www.garmin.com - About GPS link for a good explanation of the technology. The bottom line is that typical civilian GPS units are accurate to within 60-225 feet, according to Garmin.

At 120 mph you are traveling 10,560 ft/min. 60 feet more or less is a difference of .0057 (10,500/10,580), or + or minus 0.7 mph. If the error was 225 feet, the speed would be off by + or - 2.5 mph. The figures work out about the same at 60 mph or 180 mph.

I would not class 60-225 ft incredibly accurate in any way shape or form

Car and motorcycle speedos are often as much as 10% out.. 12 mph in your example above where a GPS has a 'possible' 2.5 mph error.

GPS speed readings have to be made over averages.. So again, any errors in the location are averaged out over the multiple readings it makes over the km.. It doesnt take a reading at the start and end points and calculate a speed, thereby creating your possible errors.. It takes multiple point readings as it goes, averaging a speed as it averages out locations and that averages out any geo location errors.

I would say a GPS reading to be far more accurate than a speedo.. However that comes with the caveat that its an average speed reading of your most recent distance, they are very accurate while you travel at a fixed speed and very inaccurate as you accelerate and decelerate..

Vehicle speedometers are quite consistent. I have compared the speedometers on at least five different vehicles while using my GPS. The vehicles were ALL reading about 5 KPH fast at 95 KPH speed. The speedometer read 95 and the GPS read 90. It would appear that the manufacturers purposely calibrate the speedometer to read about 5 percent fast. The kilometer markers along highways are usually quite precise if you want to do a manual check.

Vehicle speedometers are quite consistent. I have compared the speedometers on at least five different vehicles while using my GPS. The vehicles were ALL reading about 5 KPH fast at 95 KPH speed. The speedometer read 95 and the GPS read 90. It would appear that the manufacturers purposely calibrate the speedometer to read about 5 percent fast. The kilometer markers along highways are usually quite precise if you want to do a manual check.

Agree.. Every motorcycle I have tested the speedo read fast.

Around 8% on most for me.

Given that a GPS has to be incredibly accurate measuring time (to work out where the satelites are) and will usually be accurate to a few metres, the speed measurement must be very accurate if calculated over a reasonable distance (eg 1 km or so when errors in the distance measurement will be very small as a percentage of the distance travelled. So must be much more accurate that than the speedo.

I think also that speedo is based on wheel revolutions? Therefore as tyres wear speed measured will differ.

HOW ACCURATE IS THE SPEED READING?

If you are completely stopped, Selective Availability (the "noise" put into the GPS by intent to prevent military use by unfriendly folks" --See: http://joe.mehaffey.com/sabysam.htm ) causes appearent motion despite your GPS being at a fixed location. Typically this speed is in the range of zero to about 3mph (5kph)

Take a look at the Garmin site, www.garmin.com - About GPS link for a good explanation of the technology. The bottom line is that typical civilian GPS units are accurate to within 60-225 feet, according to Garmin.

At 120 mph you are traveling 10,560 ft/min. 60 feet more or less is a difference of .0057 (10,500/10,580), or + or minus 0.7 mph. If the error was 225 feet, the speed would be off by + or - 2.5 mph. The figures work out about the same at 60 mph or 180 mph.

I would not class 60-225 ft incredibly accurate in any way shape or form

You've just topped GaryA in GPS related posts that are well off the mark! Selective Availability was removed from the system in year 2000 by the Clinton administration. Are you getting all you facts from some historical archives. Suggest you clear your web browser cache! Civilian GPS accuracy has come along way since then. The second post / answer is closer to the mark.

Given that a GPS has to be incredibly accurate measuring time (to work out where the satelites are) and will usually be accurate to a few metres, the speed measurement must be very accurate if calculated over a reasonable distance (eg 1 km or so when errors in the distance measurement will be very small as a percentage of the distance travelled. So must be much more accurate that than the speedo.

I think also that speedo is based on wheel revolutions? Therefore as tyres wear speed measured will differ.

HOW ACCURATE IS THE SPEED READING?

If you are completely stopped, Selective Availability (the "noise" put into the GPS by intent to prevent military use by unfriendly folks" --See: http://joe.mehaffey.com/sabysam.htm ) causes appearent motion despite your GPS being at a fixed location. Typically this speed is in the range of zero to about 3mph (5kph)

Take a look at the Garmin site, www.garmin.com - About GPS link for a good explanation of the technology. The bottom line is that typical civilian GPS units are accurate to within 60-225 feet, according to Garmin.

At 120 mph you are traveling 10,560 ft/min. 60 feet more or less is a difference of .0057 (10,500/10,580), or + or minus 0.7 mph. If the error was 225 feet, the speed would be off by + or - 2.5 mph. The figures work out about the same at 60 mph or 180 mph.

I would not class 60-225 ft incredibly accurate in any way shape or form

You've just topped GaryA in GPS related posts that are well off the mark! Selective Availability was removed from the system in year 2000 by the Clinton administration. Are you getting all you facts from some historical archives. Suggest you clear your web browser cache! Civilian GPS accuracy has come along way since then. The second post / answer is closer to the mark.

People are entitled to believe what they want and I am entitled to believe what I know. I trust my GPS. I KNOW it is very accurate when driving a constant speed in a straight line.

My apologies Gary, you comments about speed are indeed accurate. Same as my own experience. Sorry for confusion.

I would say a GPS reading to be far more accurate than a speedo.. However that comes with the caveat that its an average speed reading of your most recent distance, they are very accurate while you travel at a fixed speed and very inaccurate as you accelerate and decelerate..

I agree with this

I have seen the same results, 2 different cars, 3 different GPS's......all exactly the same, car speedos shows consistent 5 kph faster than the GPS.

Hi.

Add myself here - Isuzu D-Max, Garmin Nuvi 205. At 120 km/h on the navi the truck's speedo shows closer to 130 km/h.

Haven't tried it on my bike yet however i got a digital (bicycle) speedo there too and those are already ridiculously accurate - shows me that my Yamaha speedo lies like Pinocchio, at indicated 100 i go 95, at indicated 140 i go 131 and at indicated 200 i go 173 That's "speedometer tuning" for the bike enthusiasts to have something to brag about - "yeeeeah i cleared the meter" (possibly having 17" wheels instead of stock 18" on top of it).

Best regards....

Thanh

The BASICS: We get L1 data from the satelites, part of L2 reserved for military. The US government "alters the quality" of L1 data.... so it's not "accurate".

Just to share (http://www.brighthub.com/electronics/gps/articles/43189.aspx?p=2):

Factors Contributing to the Accuracy of Most Reliable Portable GPS

Selective Availability: The US controlled GPS system sends signals at two different frequencies, L1 for civilians, and L2 for military only. Other than reserving access of L2 for military, the US Defense system also alters the quality and bandwidth of L1 signals at regular intervals. This affects the accuracy of GPS considerably. The workaround for this is to use Differential GPS. Otherwise too, the DGPS helps in offering greater accuracy of GPS. The most reliable portable GPS devices employ DGPS in conjunction with WAAS (see next point) to get around the problem of S/A.

Spatial Decorrelation: This is the degradation of GPS signals as they travel from satellite to the GPS device. To achieve better accuracy of GPS for faintest signals, the handhelds use several techniques. The most reliable portable GPS contain a minimum of two receivers to incorporate WAAS (Wide Area Augmentation System), which helps in providing better accuracy of GPS even if the signal is very weak or contains unclear data.

So what are you trying to say? Are you trying to say that a vehicle speedometer is more accurate than the commonly available GPS units? When my two GPS units, a 60 CX and a nuvi 750 read exactly the same and my vehicle speedometer reads reads 5 KPh hour faster, that the vehicle speedometer is better? Maybe you think that both are wrong?

My Honda Airblade speedo reads 80km/h when my satnav reads 65km/h.

The GPS tell truth, Honda lie like cheap Japanese watch!

So what are you trying to say? Are you trying to say that a vehicle speedometer is more accurate than the commonly available GPS units? When my two GPS units, a 60 CX and a nuvi 750 read exactly the same and my vehicle speedometer reads reads 5 KPh hour faster, that the vehicle speedometer is better? Maybe you think that both are wrong?

Weellll... i'm thinking that I'm not going to pick a fight with you over this.. just not worth it. and I was in no way implying that you are wrong... just quickly read the topic and decided to contribute positively as a forum member.

But let's have a civilized discussion and share thoughts / ideas / technical points:

Car speedometers are heavily dependant on wheel diameter, I'm not going into specifics of cables, sensors, just because things can get hairy and quite boring and more importantly I'm not a automotive engineer (I've taken apart and put together in running order an Amercian V8).... BUT.. most if not all cars get their speed input from the wheel and tire/tyre. Tire diamater changes according to speed, and not all tires are made equall.... so the diameter can vary from manufacturer to manufacturer... hence.... not so accurate speedometer.

GPS units, they have an error rate.. The error rate is NOT from the GPS units but from the satelites themselves. We get L1 and part of L2 data... both skewed by the military for obvious reasons. I've been playing with GPS units back from... late '90s?... Now I'm using a Nuvi 310 and a Nuvi 1460 and a bluetooth unit I picked up at Pantip to use with my notebook. I'm not saying that I'm an expert,.. but far from being a noobie. I actually never tried to run my GPS units side by side, but this I do know... they don't display information on a real time basis... and there is an error rate. This error rate is based on a whole bunch of factors that I'm not going to get into.. besides.. you can get this info on google and wiki. If there is an error rate, then it's not exactly accurate..is it?

That said.. I believe that both the speedometer and GPS units are wrong.... your's, mine, and jane's.... all wrong.. including mine.. and my car.... an my bicycle.

The only way to get the best and most accurate GPS units would be to enlist for the US or UK military and be issued a military grade GPS unit.

I agree that most things in life are not perfect. That said, the margin of error is so small as to not be important. Even an error of plus or minus two percent is insignificant.

If I were planning to put a guided bomb down a chimney, then I'd be concerned.

I've been playing with GPS units back from... late '90s?...

Dude, where were you on 2May 2000? Clinton switched off S/A on that date and civilian L1 band GPS signals significantly improved. Trust me on this one as I am in the business. The web ref you gave is wrong to imply S/A is still a factor.

I've been playing with GPS units back from... late '90s?...

Dude, where were you on 2May 2000? Clinton switched off S/A on that date and civilian L1 band GPS signals significantly improved. Trust me on this one as I am in the business. The web ref you gave is wrong to imply S/A is still a factor.

2000?.. i think I was at Gary A's state.

No doubt that turning off the S/A improved accuracy.. at best my Nuvi can give me a 5 meter accuracy rate. I'm guessing that's a overall 10 meter / 32 feet diameter? In terms of civilian use, it's good enough... but for search and rescue.. no dice... but still better than nothing. And if for military use.. i think they can get a resolution of less than 1 meter.

But hey.. i love GPS.

Yes, your comments are pretty spot on. In days pre-S/A accuracies were only of the order of 100m. Now 10m is average but ymmv. Cannot comment on the military stuff as I don't know, but civilian use can get down to millimeters with phase resolution results post processed or real time such as RTK systems.

GPS units, they have an error rate.. The error rate is NOT from the GPS units but from the satelites themselves. We get L1 and part of L2 data... both skewed by the military for obvious reasons. I've been playing with GPS units back from... late '90s?... Now I'm using a Nuvi 310 and a Nuvi 1460 and a bluetooth unit I picked up at Pantip to use with my notebook. I'm not saying that I'm an expert,.. but far from being a noobie. I actually never tried to run my GPS units side by side, but this I do know... they don't display information on a real time basis... and there is an error rate. This error rate is based on a whole bunch of factors that I'm not going to get into.. besides.. you can get this info on google and wiki. If there is an error rate, then it's not exactly accurate..is it?

Yes they have an error rate.. and when doing single point readings the error is important to be aware of..

However in calculating average speed, the GPS uses multiple readings, and averages out those readings, hence the errors, if they are truly random, will average out also. Hence the GPS speed calculation, by averaging out the errors, is very accurate.

People are entitled to believe what they want and I am entitled to believe what I know. I trust my GPS. I KNOW it is very accurate when driving a constant speed in a straight line.

Plus 1

I'll believe the GPS before I believe the speedo