Not really a tech problem but no idea how to get a definitive answer

In my experience a speedo will display a higher speed compared to a sat nav readout

My question is where does the sat nav get the speed that is displayed on the unit and is it more accurate than the speedo? - standard size wheels/tyres so no mods to affect the speedo reading

 

Most modern cars have optimistic speedo's. My Toyota truck and my wifes Subaru are out 5-10kph at 100kph. My older cars (84 and 86) are close to spot on. I think modern manufactures just want not to get involved in court cases and disputes so they default to slightly slower.

Sat nav uses GPS signals to determine your position, which it continually samples. Change in position (distance) / time =speed. I would trust any GPS based system over a modern car speedo for accuracy. You can also download an app (there are many) for your smartphone, which I find handy for an old 1960 car I have which has a grossly inaccurate MPH speedo, that I need to relate to kph speed limits.

 

Thanks for the answer Steve - I thought the gps would be a bit more accurate but wasn't too sure how often the send/receive data worked - so it's pretty much every second to get a good reading

I agree that modern cars actual speed is a bit lower than the speedo so that people don't go over limits by accident - I was quite pleased when I hit 150 mph in my old Kia but then saw the gps was only saying 139

The Jag is so smooth that 10 minutes after I started driving it felt like 40 mph and looked down and saw I was doing 80 on the clock - normally you can tell how fast you are going just by vibration and revs but that's a bit hard with a 3.0 automatic

 

Hi Dave

If I remember correctly commercial GPS devices normally range in sampling from 1-20 times per second.

139 in a Kia :-) living on the edge!

 

It was a Magentis 2.5 V6 and not one of the little ones

1-20 times a second? - that just blew my mind imagining all the cars on the planet with gps and the amount of data transmitted - no wonder you can't get a signal on your phone

 

That's not how GPS works. Data is only received, nothing is transmitted. It's similar to listening to a radio signal. Your reception doesn't depend on how many other people are listening. A typical consumer grade GPS unit is 'listening' to anywhere from 3 to 10 or more satellite signals at any given time. The signals are all sent simultaneously and the GPS unit computes the slight difference in time it took to arrive (according to distance) and calculate the precise position on the planet. The more signals it recieves the more accurate the calculation.

Although it calculates 1-20 times per second the output display is set to update only once per second or so. Our brainds are too slow to deal with anything much more frequent.

Car speedos have always been inaccurate but without GPS how were we to know?

Here's a discussion where this poor chap took the news quite badly:

https://www.jaguarforums.com/forum/x...2/#post1602114

 

:-) they are very busy little devices when you think about what they are achieving in such a small package while glued to a windscreen on a 40C day or a -20C day.