A car that genuinely under reports speed...
Discussion
I recently had the displeasure of one of those MG-badged Chinese cars, a ZS specifically, and had an unusual experience... The speedo under reported by 1mph at ~50mph.
I was using the inbuilt satnav, which decided to crash. So, I pulled over, fired up Google maps and carried on my way. Sat on cruise at a car-reported steady 50mph, but GPS revealed my speed to be 51mph.
I didn't investigate further to see if a previous customer had played with MMI settings, but it's worth noting for those who try to maximise their use of the margins before speeding enforcement occurs!!!
I was using the inbuilt satnav, which decided to crash. So, I pulled over, fired up Google maps and carried on my way. Sat on cruise at a car-reported steady 50mph, but GPS revealed my speed to be 51mph.
I didn't investigate further to see if a previous customer had played with MMI settings, but it's worth noting for those who try to maximise their use of the margins before speeding enforcement occurs!!!
donkmeister said:
I recently had the displeasure of one of those MG-badged Chinese cars, a ZS specifically, and had an unusual experience... The speedo under reported by 1mph at ~50mph.
I was using the inbuilt satnav, which decided to crash. So, I pulled over, fired up Google maps and carried on my way. Sat on cruise at a car-reported steady 50mph, but GPS revealed my speed to be 51mph.
I didn't investigate further to see if a previous customer had played with MMI settings, but it's worth noting for those who try to maximise their use of the margins before speeding enforcement occurs!!!
Have you thought about speaking to Hollywood about the screenplay? I was using the inbuilt satnav, which decided to crash. So, I pulled over, fired up Google maps and carried on my way. Sat on cruise at a car-reported steady 50mph, but GPS revealed my speed to be 51mph.
I didn't investigate further to see if a previous customer had played with MMI settings, but it's worth noting for those who try to maximise their use of the margins before speeding enforcement occurs!!!
It's possible for GPS to read high.
I've seen GPS's bolted to large concrete buildings indicate a few MPH.
If the speed is rounded to 1mph, it's not exactly a big deal for one indication to be 50 and the other 51.
Most speedo's of course indicate a few MPH faster than reality.
Tyres can vary!
I had a trail bike where the speedo was programmable for different tyre sizes, like a pushbike speedo almost.
I've seen GPS's bolted to large concrete buildings indicate a few MPH.
If the speed is rounded to 1mph, it's not exactly a big deal for one indication to be 50 and the other 51.
Most speedo's of course indicate a few MPH faster than reality.
Tyres can vary!
I had a trail bike where the speedo was programmable for different tyre sizes, like a pushbike speedo almost.
Alex_225 said:
I always assumed satnav was 100% accurate.
It might not be if you go through places with poor reception (concrete canyons, forests, etc). It will also work on the basis of a straight line between each "fix", so could be out on hairpins for example, while on steep hills the satnav will take no account of the slope as it merely "sees" a plan view of the terrain.Penny Whistle said:
Alex_225 said:
I always assumed satnav was 100% accurate.
It might not be if you go through places with poor reception (concrete canyons, forests, etc). It will also work on the basis of a straight line between each "fix", so could be out on hairpins for example, while on steep hills the satnav will take no account of the slope as it merely "sees" a plan view of the terrain.
17 inch wheels with 225/50 17 tyres fitted., for example, will have a diameter of 656mm.
I have assumed that the tyres are brand new, so 8mm of tread.
Wear those tyres down to 2mm, and you have 12mm less diameter.
12/656 = 1.83%, at 50 mph this 0.915 mph.
So it depends for what point in the tyre life the speedo is calibrated.
Then if you stand 5 different manufacturers' brand new 225/50 17 tyres on their treads, side by side, the first thing the accurate human eye notices is that they actually vary in height by way more than 1.8%.
So you will be doing well if your speedo is dead accurate, given the variation in tyres.
But I do agree with the OP - we have grown accustomed to German cars being set to read about 3 MPH high to give a margin of safety when we are gambling on not being just 1mph into the prosecute zone in the 15 miles of 50 mph limit in our favourite motorway roadworks.
I have assumed that the tyres are brand new, so 8mm of tread.
Wear those tyres down to 2mm, and you have 12mm less diameter.
12/656 = 1.83%, at 50 mph this 0.915 mph.
So it depends for what point in the tyre life the speedo is calibrated.
Then if you stand 5 different manufacturers' brand new 225/50 17 tyres on their treads, side by side, the first thing the accurate human eye notices is that they actually vary in height by way more than 1.8%.
So you will be doing well if your speedo is dead accurate, given the variation in tyres.
But I do agree with the OP - we have grown accustomed to German cars being set to read about 3 MPH high to give a margin of safety when we are gambling on not being just 1mph into the prosecute zone in the 15 miles of 50 mph limit in our favourite motorway roadworks.
For an apparent over read, I can get this when on hills.
You have the cruise set to a gps calibrated 50mph on the flat, but the gps readout will vary when going up and down hills.
It may appear to be over temporarily, but you are still doing the same speed on the speedometer.
Gps isn’t accurate unless on a flat level road.
FYI the worst car I had for under reading was a Toyota Audis hybrid. At an indicated 70mph you were only doing 63mph. Complained to Toyota but they said no adjustment was possible.
You have the cruise set to a gps calibrated 50mph on the flat, but the gps readout will vary when going up and down hills.
It may appear to be over temporarily, but you are still doing the same speed on the speedometer.
Gps isn’t accurate unless on a flat level road.
FYI the worst car I had for under reading was a Toyota Audis hybrid. At an indicated 70mph you were only doing 63mph. Complained to Toyota but they said no adjustment was possible.
Whataguy said:
For an apparent over read, I can get this when on hills.
You have the cruise set to a gps calibrated 50mph on the flat, but the gps readout will vary when going up and down hills.
It may appear to be over temporarily, but you are still doing the same speed on the speedometer.
Gps isn’t accurate unless on a flat level road.
FYI the worst car I had for under reading was a Toyota Audis hybrid. At an indicated 70mph you were only doing 63mph. Complained to Toyota but they said no adjustment was possible.
That’s over-reading. You have the cruise set to a gps calibrated 50mph on the flat, but the gps readout will vary when going up and down hills.
It may appear to be over temporarily, but you are still doing the same speed on the speedometer.
Gps isn’t accurate unless on a flat level road.
FYI the worst car I had for under reading was a Toyota Audis hybrid. At an indicated 70mph you were only doing 63mph. Complained to Toyota but they said no adjustment was possible.
And might be explained by you hybridising an Audi with a Toyota?
I tend to find on Mercedes that the speedo does not begin to under-read until you pass 50mph. That's on a few different mercs. I've also seen it on a BMW. At 50mph barely any divergence, by the time you get to 60mph some significant difference starts showing... from there the divergence increases exponentially.
Could be the manufacturers policy and that it might be different from one to the other. Could be typical of the country where the car is produced... say it is like this in all German cars for instance.
Could be the manufacturers policy and that it might be different from one to the other. Could be typical of the country where the car is produced... say it is like this in all German cars for instance.
My Nissan X Trail over-reads by 10%, so an indicated 77 is actually 70.
I am supposed to be a responsible senior manager, so whenever I have a member of staff in the passenger seat I have to explain this to them, or they think I am driving ike a lunatic. Not helped by the speedo being dead centre of the dash. Why they did that I have no idea, I can only think that the designer liked symmetry.
I suspect the previous owner either put smaller wheels on or downsized the tyres..
I am supposed to be a responsible senior manager, so whenever I have a member of staff in the passenger seat I have to explain this to them, or they think I am driving ike a lunatic. Not helped by the speedo being dead centre of the dash. Why they did that I have no idea, I can only think that the designer liked symmetry.
I suspect the previous owner either put smaller wheels on or downsized the tyres..
Edited by QBee on Tuesday 18th July 09:44
ingenieur said:
I tend to find on Mercedes that the speedo does not begin to under-read until you pass 50mph. That's on a few different mercs. I've also seen it on a BMW. At 50mph barely any divergence, by the time you get to 60mph some significant difference starts showing... from there the divergence increases exponentially.
Could be the manufacturers policy and that it might be different from one to the other. Could be typical of the country where the car is produced... say it is like this in all German cars for instance.
All 3 of the Mercedes we have had have been consistent in over-reading by 1.5 mph through to about 50, then 2 mph from there on up to slightly illegal speeds.Could be the manufacturers policy and that it might be different from one to the other. Could be typical of the country where the car is produced... say it is like this in all German cars for instance.
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