For those with a 500X and Fiat's SAT NAV then when you get a chance I would appreciate if you could do a simple test(s) and report back.

How accurately does the SAT NAV speed match the vehicles speedometer. Lower?, higher?, exactly the same? and is this at just one speed or over an entire range, say 20mph to 70+mph

If you don't have Fiat's integrated SAT NAV but do have you own Garmin/TomTom/Phone etc. then likewise how do the SAT NAV speed correlate to vehicle speedometer speeds?

Don't forget to use the digital dash speed for clarity. The speedo needle will be accurate but there will be parallax issues to address.

Don’t think my integrated actually displays current speed. It is the 5 inch tomtom one.
Are you looking for accuracy of the satnav or accuracy of the speed display on the dash?

Pedrolami said:

Don’t think my integrated actually displays current speed. It is the 5 inch tomtom one.
Are you looking for accuracy of the satnav or accuracy of the speed display on the dash?

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Same here.
Was actually thinking of posting on here cos I couldn't find how to do it.
Maybe I'm just thick?
My old Garmin shows speed, so I figured that the built in TomTom would too.
If it doesn't, would seem to be a backwards step unless, as other threads have suggested, the sat nav speed is somehow linked to the speedo display

My 500X has a 6.5 inch screen and as far as I can tell doesn't show the road speed, that is unless I have missed something.

It does/will display speed.

Info is on another page/selection.

I'll check "where" next time I'm in my car but for now I've had eye surgery so could be a few days or more before I'm back in the car.

The 5" displays the max speed of the bit of road your on. So 30, 40, 70 etc. It goes red if you exceed the maximum speed for your bit of road and seems very accurate. My car speedo displays a speed figure about 8% higher than the nav unit, i.e. roughly 32 in a 30, 75 in a 70.
My separate Garmin matches the Tom Tom nav in the car.

On the 5 inch unit you can see the GPS speed by going to "Settings" and then "GPS Status".

I'm sure I've had it displayed elsewhere but am unable quantify at this time.

I have a tomtom start on windscreen and it shows about 2mph to 3mph less than the Fiat digital speedo reading. It looks like the Fiat speedo reads 2 to 3 mph over at all speeds. I’ve also checked with my IPhone speed app and it’s the same. So actual speed is 67 mph but Fiat speedo shows 70 mph.

Pedrolami said:

Don’t think my integrated actually displays current speed. It is the 5 inch tomtom one.
Are you looking for accuracy of the satnav or accuracy of the speed display on the dash?

Click to expand...

On the 5 inch unit you can see the GPS speed by going to "Settings" and then "GPS Status".

I'm sure I've had it displayed elsewhere but am unable quantify at this time.

I'm looking for two things:

1) Accuracy of the vehicles speedometer

2) Evidence of any link between them

As for Sat Nav speed accuracy then correctly functioning commercial Sat Navs are accurate to about 0.1mph so can be relied on.

I just took a long road trip and checked the odometer against a app on my phone and also the "mile marker" signs on the highway. My odometer is off by not quite two percent. Now my tires are brand new and slightly over inflated. As the tire wears down and becomes smaller, the amount of error will lessen and the indicated mileage will be more correct.

A less than two percent error isn't that bad. I owned a Toyota that was off by almost three percent and an old Cadillac that was off by nine.

Was out earlier and would agree that's where it is, so thanks for that.
Any idea if you can get it show alongside the routing?
I was driving so couldn't mess around with it; better (looking) half gets a bit worked up if I don't concentrate on the driving............she is of course correct, as always!

s130 said:

On the 5 inch unit you can see the GPS speed by going to "Settings" and then "GPS Status".

I'm sure I've had it displayed elsewhere but am unable quantify at this time.

Click to expand...

Was out earlier and would agree that's where it is, so thanks for that.
Any idea if you can get it show alongside the routing?
I was driving so couldn't mess around with it; better (looking) half gets a bit worked up if I don't concentrate on the driving............she is of course correct, as always!

s130 said:

On the 5 inch unit you can see the GPS speed by going to "Settings" and then "GPS Status".

I'm sure I've had it displayed elsewhere but am unable quantify at this time.

I'm looking for two things:

1) Accuracy of the vehicles speedometer

2) Evidence of any link between them

As for Sat Nav speed accuracy then correctly functioning commercial Sat Navs are accurate to about 0.1mph so can be relied on.

Click to expand...

There's no link between them as per previous threads.

Odometer values are loosely related to indicated MPH/KPH.

The looseness is very much related to how odometer and speed values are derived.

In the old days odometer distances were fixed gearing drives to a numerical counter. Speed indication was a "rate of rotation" in a "drag cup" against a coil spring thus determining a needle deflection/rotation.

These days things are a little more refined. However it is not uncommon for vehicle systems, eg. ABS, Body Computer, Engine ECU, etc. to have different values logged.

This is a common question/issue with the likes of MultiECUScan where people are finding different values in different systems and then wondering if there is a vehicle issue, have they a clocked car, etc.

robotdancer said:

There's no link between them as per previous threads.

Click to expand...

Well I don't agree or disagree with you.

Let me be clear and respectful here.

I have a shed load (45 years+) of Fiat technical experience, was a Fiat Product Engineering feedback agent, and still have some valuable and treasured, but diminishing links, with Fiat UK technical support.

That aside for this question of is there a link between the on-board Sat Nav and the Body/Dash computer node regarding speed alignment/calibration it is not burning technical issue that drives me to rattle my Fiat contacts. I reserve such contacts for major issues.

That does not stop me investigating, researching etc. ANY technical type topic. This is how and why we all take steps forward.

You have said "their is no link" but not provided detailed technical supporting information.

I said my Speedo (digital) exactly matches the on-board Sat Nav speed. I have no detailed technical supporting information a link between Sat Nav and Dash node.

This is why we have to enquire, drill down, etc.

Having found the bit that reports the speed, Driving Home from work last night I set the cruise to 30mph, the display said 29mph and the gps reported 27mph.

I personally would suspect that there is no connection between either, we have the Mersey tunnel which has no gps signal while inside it. The speedo doesn’t twitch or change when entering and shows a constant speed when the cruise is enabled.

Pedrolami said:

I personally would suspect that there is no connection between either, we have the Mersey tunnel which has no gps signal while inside it. The speedo doesn’t twitch or change when entering and shows a constant speed when the cruise is enabled.

Click to expand...

Tunnels do not present an issue for good quality Sat Nav systems. When GPS system signal is lost then they use inertial data to keep the system up to date.

If there were a link between the Sat Nav and speedo, which now looks unlikely, then I would expect it to be a periodic check and self calibrate.

Meanwhile the differing speed values are looking interesting in their own right.

When I'm able to I'll cross check the 500Xs vehicle speed values across the various nodes. On the 500X the following nodes have vehicle speed - ECU,ABS,Airbag,Steering,Gearbox,Dashboard,Body & Parking Sensors. Many of these will be just a common CAN signal value that is being propagated between the nodes. ABS vs possibly Dash or Gearbox might be interesting.

Another one is what happens in Traction Plus Mode. Here the system is supposed to keep optimum drive to each wheel in poor grip scenarios. Some videos show the front wheels rotating a lot faster than the passive rear ones. I wonder what happens in this case regarding vehicle speed measurement? I'm assuming gearbox final drive, before differential is the driver. If that is the case then the speedo will indeed show elevated speed.

s130 said:

Tunnels do not present an issue for good quality Sat Nav systems. When GPS system signal is lost then they use inertial data to keep the system up to date.

Click to expand...

I can agree with that. I work for the ambulance service and the old navigation devices we used to have appeared to be hard wired into the vehicle to record speed. When in the tunnel and coming to a stop, the device would show correctly that we were slowing down.
However, we have moved over to 'improved' Garmin devices which are your bog standard ones designed for commercial vehicles. Other than permanent power connected to the data terminal, this doesn't seem to get any other information.
When entering the tunnel, it displays the speed we entered regardless of what happens.

I have never really paid that much attention to the one in my car to notice if it does anything similar, however the next time I make the journey I will see if it is affected by the speed of the vehicle.

You also mention about the Traction Plus Mode. When working for another emergency service, I had dealings with a car that was involved in a collision, with the speedo stuck at 47mph in a 30 limit. Traffic concluded that it was not unreasonable that as the car collided, one or both front wheels left the road and with the drivers foot still on the accelerator, the speed of the engine increased resulting in what appeared to be an incorrect speed at the point of impact.

Ok, checked mine today, only had the opportunity to do it at the lower speeds. I didn't try to read the analogue speedo as well, although I've noted in the past it matches the digital one pretty well. My results are as follows:
Speedo = 20mph, GPS = 19mph
Speedo = 30mph, GPS = 28mph
Speedo = 40mph, GPS = 38mph
Speedo = 50mph, GPS = 47mph
So, a fairly consistent offset. Although using the GPS to recalibrate the speedo is a nice idea, I don't think the car does it. Assuming the GPS indicated speed is valid (which it should be), the speedo is commendably accurate, given that my tyre tread depth is several mm above the legal minimum. I suspect the speedo is set up to not under-read (illegal) at minimum tread depth.
As a retired (non-automotive) engineer, I enjoy learning all the little details of modern cars, although sadly it's usually difficult to find definitive information. I really would like to see the comparative power and torque curves for the three driving "moods" for instance.