GPS vs. Speedometer

cregser

I got myself a shiny Garmin Nuvi 310D yesterday and noticed it has it's own speedometer. But the thing is about 5km/hr slower than the one on my car.
I've always heard that speedometers on cars weren't great but 5km/h difference at 50km/h is pretty big. At 120km on my car it says I'm doing 113km. You'd think this would be a blessing in disguise but I got a speeding fine 2 months ago :rolleyes:.

Is a GPS pretty much gospel when it comes to telling you your speed or is some sort of Einstein special relativity equation needed that my satnav is skimping on?

Doodah7

I also have a 310D and is exactly the same as yours as the Sat Nav is 5km slower than the speedometer in the car at 120km/h.

Have to assume it is the device so...

auggie2k

Picture it this way... the car is connected to the speedo... the GPS is going miles up and back down to the car... I expect a few delays or errors. I'd trust the car.

doolox

A couple of nightmare scenarios come to mind.
I was listening to a radio program about the future applications of mobile phones and GPS systems whereby the road safety authorities will be able to spot check a cars speed and fine the owner without having to stop you in a speed trap or measure your speed externally. The authorities will use signals from the mobile phone to track all cars on GPS and issue fines accordingly. The system is sophisticated enough to include information about 50 and 80 Km/ph zones etc. First you will know of it is a fine in the post.......
The next nightmare scenario is feedback from a central control system to override a drivers input to the car, now mostly by electronic control units to an engine management system to stop people speeding altogether.......
If you have ever watched two lorries trying to overtake each other with speed controllers fitted you will get the idea. It will make safe overtaking virtually impossible.
Third nightmare scenario is a machine readable licence which the user will have to insert in order to drive the car. Lose the licence lose your mobility.

jor el

I once read that speedometers in cars are calibrated down slightly, otherwise when you think you're doing 100kph, you could actually be breaking the speed limit and therefore the car manufacturer could be liable since you have no other way to determine your speed.

I've noticed a roughly 5% discrepancy between my speedometer and the GPS too and I'd say the GPS is more accurate.

hughowen

The GPS is definately more accurate, check it out on a long drive like I did in France going exactly 120km p/hr on my car clock but was not getting 120km per hour in distance. The GPS said I was going 114km p/hr and that was closer to the distance I travelled in 2 hours. 224k/m
I was also on cruise control so, no slowing down
Garin Nuvi 310d

Hugh

Fnz

IIRC car manufacturers can get into a bit of trouble if the speedo informs the driver that s/he is going slower than s/he actually is.

Car manufacturers overcompensate a little to ensure that this doesn't happen.

robinph

The GPS is more accurate than the car speedo once you average it over a distance, but it can be wildly out if you just take the speed as calculated between two points.

Someone in the US is challenging a speeding ticket they got though as their parents had fitted a tracking GPS to his car that showed he was doing under the limit:

The Register

MickH503

I have a TomTom One and it is same as OP above - about 6km difference between car and GPS. The interesting thing is that my car can tell me my average speed, so if I'm doing a constant 120km/h on car speedo and reset the average speed reading, it'll then tell me that my average speed is 114km/h ... so that would imply that the car manufacturer (Ford in my case) sets up the speedo to read higher than actual speed on purpose. :eek:

Krusty_Clown

The GPS is more accurate, except when detecting changes in speed (acceleration, deceleration), for which the speedo is more acurrate.

Better off trusting whichever is showing the slower speed.

robinph

Krusty_Clown wrote: »

Better off trusting whichever is showing the slower speed.

You mean the one showing the faster speed.

Alun

MickH503 wrote: »

... so that would imply that the car manufacturer (Ford in my case) sets up the speedo to read higher than actual speed on purpose. :eek:

Yes, they do .. it's a well known fact, as do many other car manufacturers. It's all to do with an EU directive that specifies the range of errors a speedo may display. They can overread by a certain amount but can never, ever underread, therefore the manufacturers build in a constant percentage overread to ensure this never happens, due to, say, tyre wear or underinflated tyres.

If you have a newer Ford, you can put the on-board computer into a diagnostics mode where (amongst a raft of other things) you can display the speed. It's no coincidence that this speed agrees almost exactly with the GPS, whilst the normal speedo overreads by anything up to 5% or so.

It's got nothing to do with the mechanism of the speedo either. Modern car speedos aren't connected to the gearbox with a cable like in the 'old days' .. there's a sensor in the gearbox which feeds back electronic pulses to the cluster computer that translates this into signals to be sent to a stepper motor that drives the pointer, so it's potentially as accurate as the digital readout, but the 'correction' is done in the cluster firmware.

cregser

Cool, that answers my question so. I'd pretty much garnered that information from spending more time with my GPS - i.e. more accurate but slower to react. My Dad actually reminded me of one of those speed monitors near where we live that shows you your speed - haven't tested it yet though.

Doolox, I'd say the government would be more worried about sorting out provisional drivers for the moment before introducing those nightmare scenarios. And I think the public reaction to the recent RSA laws demonstrates how touchy we are about the lawmen messin with our driving. Could happen though.

Robinph, that case is pretty interesting. But I'm sure the GPS data is not tamper proof.

Alun, that's cool. My car's a 07 Mazda3 (based on the Ford). I wonder if that has a diagnostics mode as well?

Alun

cregser wrote: »

Cool, that answers my question so. I'd pretty much garnered that information from spending more time with my GPS - i.e. more accurate but slower to react.

The 1 second delay is more down to the internal architecture of most GPS devices. More often than not, there's a separate GPS receiver connected internally via a serial link to the main computer board, squirting data over that link in NMEA or SiRF format. Generally speaking the default frequency for sending those messages is every 1 second.

Alun, that's cool. My car's a 07 Mazda3 (based on the Ford). I wonder if that has a diagnostics mode as well?

Dunno. On my Ford, you hold the button on the end of the indicator stalk in while you switch on the ignition, and hold it there for a few seconds until the display comes up with some message (can't remember what exactly, something like Diags). First it does a dashboard light test and a complete dial sweep (cool!) and then you can cycle through the various displays until you get speed. Worth a try.

311

I've checked my GPS against the speed read out in raheny ,a couple of times and the GPS is correct.

It makes cars seem more economical ,which is a bit of a rip off.

unclebill98

Must say this was bugging me. Got the Nuvi310 and yes i noticed the speed thing..... Atleast i ain't the only one.

Krusty_Clown

robinph wrote: »

You mean the one showing the faster speed.

That's right.. Just checking if anyone was paying attention.