Car speedo or satnav?

crossy

Hi
Just wondering which is more accurate. The car speed dial or the satnav?
When I do 120km/h in the car the satnav reads 116km/h

Cheers

colm_mcm

Satnav.
Speedos always over read.

SamAK

I heard that by law, speedos can over-read speed by up to 10%, but never under-read. Open to correction...

The lads on Top Gear always have satellite thingies fitted when doing speed related challenges. so that says it all really....

Supergurrier

Boost Gauge

RustyNut

My phone sat nav matches the truck tacho within 1K and tacho are calibrated to be accurate. The car is between 5 and 10 K's out depending on speed.

MuppetCheck

Sat nav.

If you're using an Android phone speed view is very handy. Drove my father's car back from Waterford today and couldn't understand how we weren't making progress until I saw how far out the speedo was using it. 93kph at an indicated 100.

GarIT

Straight road SatNav anything else Speedo.

colm_mcm

SamAK wrote: »

I heard that by law, speedos can over-read speed by up to 10%, but never under-read. Open to correction...

The lads on Top Gear always have satellite thingies fitted when doing speed related challenges. so that says it all really....

Imagine the lawsuits if you got caught speeding and the speedo said you weren't.

bazz26

Sat Nav as it uses GPS which is more accurate.

Mycroft H

.... only on the level though!

GarIT

bazz26 wrote: »

Sat Nav as it uses GPS which is more accurate.

A spedo is more accurate if calibrated properly (they are usually a bit off). A satnav will always assume you are exactly in the middle of the road (not the lane), if you aren't going straight it will be wrong.

Alanstrainor

Sat nav on a straight and level road will win every time.

Truckermal

I tested this out at the weekend using the built in Nav in my car and my Android phone and both read 120KM/PH when the speedo read 120KM/PH

dgt

SamAK wrote: »

I heard that by law, speedos can over-read speed by up to 10%, but never under-read. Open to correction...

The lads on Top Gear always have satellite thingies fitted when doing speed related challenges. so that says it all really....

We have a Daily van that reads 80kmh but according to the sat nav does 84kmh....

colm_mcm

Truckermal wrote: »

I tested this out at the weekend using the built in Nav in my car and my Android phone and both read 120KM/PH when the speedo read 120KM/PH

dgt wrote: »

We have a Daily van that reads 80kmh but according to the sat nav does 84kmh....

Bigger wheels?

DuckSlice

GarIT wrote: »

A spedo is more accurate if calibrated properly (they are usually a bit off). A satnav will always assume you are exactly in the middle of the road (not the lane), if you aren't going straight it will be wrong.

A sat nav uses triangulation and as far as the GPS part of it is concerned, which is the part that will be used to calculate your speed, it has no idea you are even on a road. It's the computer inside the satnav that plots your position on a map using the GPS co-ordinates. It makes sense in my head

dgt

colm_mcm wrote: »

Bigger wheels?

That's what the operators manual says, what's always been on it. 225/70R15c

We calculated that the for every indicated mile clocked up, it actually does 1.1 miles (miles, not km)

Italian van built in Spain is bound to have its quirks :pac:

GarIT

etxp wrote: »

A sat nav uses triangulation and as far as the GPS part of it is concerned, which is the part that will be used to calculate your speed, it has no idea you are even on a road. It's the computer inside the satnav that plots your position on a map using the GPS co-ordinates. It makes sense in my head

All the satnav knows is where you are now, where you were a the last reading, where you were the reading before that, etc, from that it works out the distance travelled between the last two points and the time taken, if it didn't calculate the curve in the road the speed reading would be off.

DuckSlice

GarIT wrote: »

All the satnav knows is where you are now, where you were a the last reading, where you were the reading before that, etc, from that it works out the distance travelled between the last two points and the time taken, if it didn't calculate the curve in the road the speed reading would be off.

That's pretty much like I said. It won't calculate the curve it will take it as a straight line from previous point to new point.

timmy4u2

SamAK wrote: »

I heard that by law, speedos can over-read speed by up to 10%, but never under-read. Open to correction...

The lads on Top Gear always have satellite thingies fitted when doing speed related challenges. so that says it all really....

Especially as they are sitting in the car on a trailer for most of the shots

wotswattage

Mycroft H wrote: »

.... only on the level though!

Is that a fact?
I'm not saying I know just wondering if anyone can say for sure either way!

I know gps takes altitude into account too so I would assume the satnav/phone measures the distance between 2 points taking lat long and altitude into account. It would surely be less complicated than converting all the readings to a flat plane then measuring??

air

The going around a bend bit can be discounted also as a source of significant errors in sat nav speed readout.
The time between subsequent readings is between 0.1s & 1s depending on the GPS module. The difference between the length of the curved path travelled and straight line distance over such a short segment is going to be negligible, even more so the faster you are going.

Truckermal

colm_mcm wrote: »

Bigger wheels?

Nope using the ones from the factory but they were part of a upgrade package.

georgefalls

http://www.telegraph.co.uk/motoring/caradvice/honestjohn/9175881/Is-a-speedo-more-accurate-than-satnav.html

crossy

Cheers everyone for the responses. Just couldnt figure it out as the satnav has to be right when it calculates the time to destination