Discussion
Probably been asked a million times already but if you have a car which shows both speed on the sat nav and your standard primary speedo in your dash brinicle which one is more trustworthy? I've seen it on many cars over the years where the different speedos give different measurements.
In favour of the sat nav the readout is in a digital format so judging exactly where the needle is pointing isn't an issue. It also does not know if you're in a plane, on a train or a boat or just walking. All it does is calculate your speed from satellites. In the car you have things like tyre wear and other calibration factors but it is a gauge which is supposed to tell you your speed so why question it?
In favour of the sat nav the readout is in a digital format so judging exactly where the needle is pointing isn't an issue. It also does not know if you're in a plane, on a train or a boat or just walking. All it does is calculate your speed from satellites. In the car you have things like tyre wear and other calibration factors but it is a gauge which is supposed to tell you your speed so why question it?
Assuming the road is fairly flat and straight and you are reasonably confident of your throttle input, then the SatNav.
I drive through 50mph road works at 55mph (if traffic is light and it it safe to do so). If you think about it, all those doing an indicated 50mph on their speedos are in reality doing 44-48mph depending on the age of the car.
I drive through 50mph road works at 55mph (if traffic is light and it it safe to do so). If you think about it, all those doing an indicated 50mph on their speedos are in reality doing 44-48mph depending on the age of the car.
vikingaero said:
Assuming the road is fairly flat and straight and you are reasonably confident of your throttle input, then the SatNav.
I drive through 50mph road works at 55mph (if traffic is light and it it safe to do so). If you think about it, all those doing an indicated 50mph on their speedos are in reality doing 44-48mph depending on the age of the car.
Shouldn't the sat nav show the right speed (assuming it is accurate in the first instance) regardless what hill or corner or throttle position you actually have as it is measuring the speed independent from the vehicle? I drive through 50mph road works at 55mph (if traffic is light and it it safe to do so). If you think about it, all those doing an indicated 50mph on their speedos are in reality doing 44-48mph depending on the age of the car.
Also I have satellite speedo apps on my mobile phone which can read from a satellite. Sometimes I like to see how fast the train is going. Was amazed the other day when some little train that looked like a greenhouse was maintaining 125mph steady speed on the way down to London. I thought you had to be on one of those pointy ones with the small windows to go at that speed.
Freakuk said:
My understanding is manufacturers will always ensure your speedo over reads, no manufacturer wants to be in a courtroom because of a speeding ticket and it be proven that the manufacturers speedo was inaccurate and cause you to speed.
UNECE regulation 39 covers this. It is not the manufacturer’s good will or other, it is a legal requirement.Speedo must display the actual speed within the range of minus 0 to plus 10% + an additional 4 kph (2.5mph)
https://unece.org/fileadmin/DAM/trans/main/wp29/wp...
Edited by Pica-Pica on Wednesday 8th March 12:09
I'd be wary of the update frequency of a GPS speedo.
If you are travelling at a steady speed it would be OK, but any sudden acceleration or deceleration and the GPS unit will lag behind the actual speed due to "satellite delay".
You just have to watch any dash cam video where the DCW has GPS speed enabled. The car can be stopped but the dash cam still shows it slowing down. Its only a second or two but it could make all the difference.
If you are travelling at a steady speed it would be OK, but any sudden acceleration or deceleration and the GPS unit will lag behind the actual speed due to "satellite delay".
You just have to watch any dash cam video where the DCW has GPS speed enabled. The car can be stopped but the dash cam still shows it slowing down. Its only a second or two but it could make all the difference.
ingenieur said:
vikingaero said:
Assuming the road is fairly flat and straight and you are reasonably confident of your throttle input, then the SatNav.
I drive through 50mph road works at 55mph (if traffic is light and it it safe to do so). If you think about it, all those doing an indicated 50mph on their speedos are in reality doing 44-48mph depending on the age of the car.
Shouldn't the sat nav show the right speed (assuming it is accurate in the first instance) regardless what hill or corner or throttle position you actually have as it is measuring the speed independent from the vehicle? I drive through 50mph road works at 55mph (if traffic is light and it it safe to do so). If you think about it, all those doing an indicated 50mph on their speedos are in reality doing 44-48mph depending on the age of the car.
ingenieur said:
Also I have satellite speedo apps on my mobile phone which can read from a satellite. Sometimes I like to see how fast the train is going. Was amazed the other day when some little train that looked like a greenhouse was maintaining 125mph steady speed on the way down to London. I thought you had to be on one of those pointy ones with the small windows to go at that speed.
A Class 158/159, which looks like a shed with a vagina on the front will easily do 95mph, and they are over 30 years old! They do sound good too
ingenieur said:
Shouldn't the sat nav show the right speed (assuming it is accurate in the first instance) regardless what hill or corner or throttle position you actually have as it is measuring the speed independent from the vehicle?
GPS gives a very accurate horizontal speed, but is not as accurate with altitude, and I doubt a car satnav will even take in to account.To make the maths easy imagine a 3-4-5 triangle. You drive up a 37ish degree hill for long enough your ground track will show 4 meters for every 5 meters of road you've covered, so the GPS speed would read say 40mph when your actual speed was 50 mph.
Triumph Man said:
A Class 158/159, which looks like a shed with a vagina on the front will easily do 95mph, and they are over 30 years old!
They do sound good too
Well maybe, but nothing like as good as a Class 68. They do sound good too
Back on topic... I'd say a GPS indication would be low (though by not that much) when ascending or descending a steep hill.
And which would be the more accurate GPS indication? My phone or a (quite old now) Garmin E-Trex that locks into four satellites (and takes a while to do it)?
Edited by Maxym on Wednesday 8th March 13:18
RizzoTheRat said:
ingenieur said:
Shouldn't the sat nav show the right speed (assuming it is accurate in the first instance) regardless what hill or corner or throttle position you actually have as it is measuring the speed independent from the vehicle?
GPS gives a very accurate horizontal speed, but is not as accurate with altitude, and I doubt a car satnav will even take in to account.To make the maths easy imagine a 3-4-5 triangle. You drive up a 37ish degree hill for long enough your ground track will show 4 meters for every 5 meters of road you've covered, so the GPS speed would read say 40mph when your actual speed was 50 mph.
"Overall, Porlock Hill is 3.94km long with an elevation gain of 366m. How steep is Porlock Hill? Well the average gradient is 9.3%. But don't let that fool you. As with most climbs in the UK the gradient is rarely constant. In the early sections of the climb, you'll tackle a maximum gradient of up to 23.5% gradient"
Porlock Hill is supposedly the steepest A road in the UK?
My car with speed info on a built-in navigation system never varies from the digital reading on the dash which in turn matches exactly the speedo dial.
When I use a plug-in Satnav there's very little discrepancy. I've even had partner call up GPS speed on iPhone to the same result. I think it's risky to assume there is big over-reading these days.
When I use a plug-in Satnav there's very little discrepancy. I've even had partner call up GPS speed on iPhone to the same result. I think it's risky to assume there is big over-reading these days.
Panamax said:
My car with speed info on a built-in navigation system never varies from the digital reading on the dash which in turn matches exactly the speedo dial.
When I use a plug-in Satnav there's very little discrepancy. I've even had partner call up GPS speed on iPhone to the same result. I think it's risky to assume there is big over-reading these days.
What car is that? When I use a plug-in Satnav there's very little discrepancy. I've even had partner call up GPS speed on iPhone to the same result. I think it's risky to assume there is big over-reading these days.
Panamax said:
ingenieur said:
What car is that?
MercCar speedos almost always over-read, but to different extents. When my car has its cruise control set to 70mph, my wife's car overtakes it with ease with its cruise control set to the same speed - it looks like the speed which shows up as 66mph on her car shows as 70mph on mine.
Compared to GPS speed (or the old fashioned way of timing how long it takes to do a mile between the motorway marker posts), the speed display in my wife's car (Leon) is only a couple of MPH above what it. My car shows a bit higher compared to normal speeds, and is quite close to the +10% permitted limit.
Rather than using GPS speed "live", it's better to calibrate the car speedo error against a few GPS runs and mile time measurements (you need a passenger in the car to do this safely). Then, using the example of my car I've got good confidence that 75mph on the clock is still below actual 70mph etc. without needing a GPS speedo running the whole time.
You need to re-do this though if you change the tyres - car speedos over-read more on worn tyres than they do with brand new ones which have a diameter about 12mm bigger. Now cars have larger wheels this is less significant than it used to be, but still does have a noticeable effect.
Compared to GPS speed (or the old fashioned way of timing how long it takes to do a mile between the motorway marker posts), the speed display in my wife's car (Leon) is only a couple of MPH above what it. My car shows a bit higher compared to normal speeds, and is quite close to the +10% permitted limit.
Rather than using GPS speed "live", it's better to calibrate the car speedo error against a few GPS runs and mile time measurements (you need a passenger in the car to do this safely). Then, using the example of my car I've got good confidence that 75mph on the clock is still below actual 70mph etc. without needing a GPS speedo running the whole time.
You need to re-do this though if you change the tyres - car speedos over-read more on worn tyres than they do with brand new ones which have a diameter about 12mm bigger. Now cars have larger wheels this is less significant than it used to be, but still does have a noticeable effect.
AndySheff said:
Just about every car I've used a GPS in - the cars speedo underreads by about 5-6 kmh. So a GPS indicated 90kmh shows 85 on the cars speedo.
The only exception to this is every Mustang I've driven. There the speedo is under by just 1 kmh.
That is the complete opposite of what the regulations say and what I have experienced in every car that I have driven.The only exception to this is every Mustang I've driven. There the speedo is under by just 1 kmh.
Car speedos are designed to read higher than the actual speed, as others have mentioned, this is to prevent you taking your car manufacturer to court when you get a speeding ticket.
And to answer the OP's question, on a flat surface at a steady speed the GPS device will be more accurate.
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