gps vs speedometer

Mandzhalas

i have tried little experiment.Just to compare speed readings from satnav/smartphone with car speedometer.

tested with 4 cars:
bmw 525,ford mondeo,opel vivaro,volvo s60

used tomtom satnav and samsung galaxy s3,galaxy s4 and huawei ascend

while i am driving car passenger beside me looking at speed readings from phone, and i compare them with car reading.
Every time when i was driving car at 100km/h(speedo reading) every phone/satnav displayed 91-93km/h

which readings where most likely to be correct?by car or gps satellite?

monkeybutter

Mandzhalas wrote: »

i have tried little experiment.Just to compare speed readings from satnav/smartphone with car speedometer.

tested with 4 cars:
bmw 525,ford mondeo,opel vivaro,volvo s60

used tomtom satnav and samsung galaxy s3,galaxy s4 and huawei ascend

while i am driving car passenger beside me looking at speed readings from phone, and i compare them with car reading.
Every time when i was driving car at 100km/h(speedo reading) every phone/satnav displayed 91-93km/h

which readings where most likely to be correct?by car or gps satellite?

The satnav is much closer to correct. The Speedo can be off by up to 10k at that speed. Confirm it by driving past a digital speed sign and it matches the satnav not the speedo. The only car I have been in that was spot on was a new clio for some reason, was bang on the money

smokin ace

Mandzhalas wrote: »

i have tried little experiment.Just to compare speed readings from satnav/smartphone with car speedometer.

tested with 4 cars:
bmw 525,ford mondeo,opel vivaro,volvo s60

used tomtom satnav and samsung galaxy s3,galaxy s4 and huawei ascend

while i am driving car passenger beside me looking at speed readings from phone, and i compare them with car reading.
Every time when i was driving car at 100km/h(speedo reading) every phone/satnav displayed 91-93km/h

which readings where most likely to be correct?by car or gps satellite?

The GPS reading are correct because the speedos of cars are set to read more than the actual speed your traveling for example if you speedo is showing 100kph your real speed might only be around 95kph or less

Yawns

GPS are more accurate than the speedo of a car usually.

Mandzhalas

so it turns out every car i drove was set to display 9km/h more tan actual speed by default

Marlow

Yep. Can be wrong with up to 10%. It's alright, as long as your speedo shows too much (as in you're driving slower than the speedo shows).

It's only illegal, if your speedo is showing less than your actual speed.

The margin on the speedos is there to accomodate for the range of tyres specified by the manufacturer of your car. Different diameters of tyres/wheels will give different speed readings.

The only thing you can rely on is GPS, if you have enough satelites in view or the readings of the likes of the speedtraps/speedsigns, if they're calibrated correctly.

Hence, if somebody is faster than you, he might not be speeding. He might just have a calibrated speedo. Trucks over 3.5t have to have a calibrated tachograph, hence accurate. So if you see them doing 95 km/h, they might only be doing 88 km/h actually, as thats where their limiter is set.

/M

L1011

Analogue speedos are very easy to misread if not looking at them dead on also. You'll usually get less of an over-read on a digital speedo, my C4 with the correct sized tyres on was +1km/h only.

scamalert

wont agree with most here,as gps do lag a good bit,where i cant see cars topping nearly 10km//h on speedo +/- .From my experience you could be up couple kms or down on gps readings,as it depends how many sats it has connected to,then weather conditions and so on,sat nav takes couple sec to respond to full stop or fast movement,where car speedometer is instantly on connected with gearbox and engine/wheels speed pedal.That said there is space for error on each reading and depending on vehicle,but if i was to choose which one to trust speddo would be best choice.

Zcott

Great for getting places, you can sit just above the speed limit and you're actually sitting right on the limit.

Del2005

scamalert wrote: »

wont agree with most here,as gps do lag a good bit,where i cant see cars topping nearly 10km//h on speedo +/- .From my experience you could be up couple kms or down on gps readings,as it depends how many sats it has connected to,then weather conditions and so on,sat nav takes couple sec to respond to full stop or fast movement,where car speedometer is instantly on connected with gearbox and engine/wheels speed pedal.That said there is space for error on each reading and depending on vehicle,but if i was to choose which one to trust speddo would be best choice.

That's why you cruise control.

Jordan537

if the speedo can be inaccurate by up to 10km, surely that overtime will give a false mileage?

CiniO

Mandzhalas wrote: »

so it turns out every car i drove was set to display 9km/h more tan actual speed by default

It's 9km/h difference at 100km/h.

It doesn't mean that if your speedo shows 40km/h then you are travelling 31.

Difference is most likely percentage. So f.e. your speed is 9% less than once indicated by speedo.

At 10km/h this will be less than 1km/h difference, while at 200km/h this will be 18km/h difference.

CiniO

Jordan537 wrote: »

if the speedo can be inaccurate by up to 10km, surely that overtime will give a false mileage?

It won't because most odometers are very accurate.

Mandzhalas

CiniO wrote: »

It's 9km/h difference at 100km/h.

It doesn't mean that if your speedo shows 40km/h then you are travelling 31.

Difference is most likely percentage. So f.e. your speed is 9% less than once indicated by speedo.

At 10km/h this will be less than 1km/h difference, while at 200km/h this will be 18km/h difference.

yes.driving at 50km/h gps shows about 48km/h

kirving

CiniO wrote: »

It won't because most odometers are very accurate.

Isn't the odometer working off the same sensor?

Jordan537 wrote: »

if the speedo can be inaccurate by up to 10km, surely that overtime will give a false mileage?

It doesn't matter anyway, since all cars will be the same within a few per cent (eg: Car A overestimates by 10%, car B overestimates by 7%). So all cars are will be within a couple of per cent of one another. Anyway, when you start getting into big mileage, how the car was looked after is far more important.

CiniO

Kevin Irving wrote: »

Isn't the odometer working off the same sensor?

It is.
But it's not like that speedmeter shows overestimated speed because sensor is not precise. It shows overestimated speed, because it's designed to do so.

Sensor can be very precise, and therefore odometres usually are also very precise.


I even tested my odometer once, and when I was travelling at around 150km/h (according to GPS - speedo was showing about 163km/h) at constant speed for an hour distance travelled and shown by odometer was exactly 150km.
That proves also that GPS speed reading are very accurate.

Marlow

CiniO wrote: »

It is.
But it's not like that speedmeter shows overestimated speed because sensor is not precise. It shows overestimated speed, because it's designed to do so.

Sensor can be very precise, and therefore odometres usually are also very precise.


I even tested my odometer once, and when I was travelling at around 150km/h (according to GPS - speedo was showing about 163km/h) at constant speed for an hour distance travelled and shown by odometer was exactly 150km.
That proves also that GPS speed reading are very accurate.

Aye, in most cars the inaccuracy is a design thing.

Take most BMWs for example. The speedo will be 10% off approx., if you however enable the digital diagnostics readout in the instrument cluster, it'll show you the correct sensor values, which are dead on.

/M

Voodoomelon

I tested my E38 before at very high speed. It was exactly 5% off.