GPS speedometer and tunnels?

GPS speedometer and tunnels?

I just read a Norwegian review of the Model S, and noticed that the speedometer is GPS based.

How will this work in a long tunnel? In Norway we drill an underwater tunnel under every fjord we can find. One close to me is 6 km long, and pretty steep both up and down. Construction will soon start on a 14 km long one.

Will the cruise control be able to keep the speed constant if set before entering the tunnel?

The main problem here is that there are speed cameras in these tunnels. Both regular ones, and average speed cameras that measure the speed all the way down...

- Erik

jbunn | October 26, 2012


I really doubt that it is GPS controlled. Much easier just to get the speed off the motor RPM or the wheel sensors. No gears to change so motor RPM unlike a car is directly related to MPH, assuming the wheels are not spinning.

More importantly, unless you have the Tech Package, you don't have GPS, so that would require two seperate ways of measuring the speed depending on Tech or no Tech package. I can't imagine they would go through the extra work.

What would be good is having the GPS know what road you are on, and looking up the local speed limit at that spot in the road, and using it for smart cruse. But again, you'd need the Tech package. Is it possible thats what the article is mentioning?

mrspaghetti | October 26, 2012


What leads you to believe it is GPS controlled?

ErikNorway | October 27, 2012

This here article, which is in Norwegian, states that there is a "GPS based speedometer":

Good luck ;)

petersv | October 27, 2012

The journalist that wrote that article is wrong... :)

Cattledog | October 27, 2012

OK, I'll give it a go since I lived in Denmark for a year - twenty five years ago - on a study abroad year and Danish is close to Norwegian. The title:

Tesla Model S: Dette er en julegave fra norske politikere

Essentially translates to:

Tesla Model S (duh): A Christmas gift from Norwegian politicians

I imagine it is referencing the large tax incentives offered there to electric cars and so a car like the Model S is really competitively priced compared to luxury alternatives.

I know all Scandinavians know English better than we know any Norse languages, so please correct my translation if it's off.

Hej hej,


jkirkebo | October 27, 2012

I doubt the speedometer is GPS-controlled, it could however be GPS calibrated automatically. That would be really nice, 100% correct speedometer all the time. Auto-calibration should be quite slow, averaging difference over several miles and slowly adjusting the speedometer until speeometer and GPS match.

mrspaghetti | October 27, 2012

@petersv +1

Sudre_ | October 27, 2012

I don't even think in the US a GPS only speedometer would be legal. The car must have an accurate odometer and that can't happen with a GPS based system.

stevenmaifert | October 27, 2012

The Nav in my LEAF displays the local speed limit, but is not connected to the cruise.

By the way Tesla, the Nav is standard equipment in all LEAFs, as is SiriusXM Satellite Radio with a 3-month trial subscription to both their entertainment and traffic service. The traffic service is slick. Displays on the Nav map and you can also select a line item breakdown of traffic in your vicinity or along your intended route. You also get three years of free data connection with AT&T for the remote apps.

ltd | October 28, 2012

Speedo is NOT GPS controller. Anyone who has driven one and sees how quickly it updates/reacts to speed shows its being driven from rpm/techo sensor.

'Commercial' GPS simply is not that 'fast' at updating (deliberately - for military reasons.

Timo | October 29, 2012

I think I read that speedometer is GPS-calibrated but once calibrated it uses motor RPM to figure out speed. Calibration happens (or should be done) whenever you change tires and is one time thing not ongoing process. Can't remember where I got this impression though. Some post dealing with tire change I suppose.

jkirkebo | October 29, 2012

Since the tires will rotate faster when they get worn I think the calibration actually should be an ongoing process. But since we charge tires two times a year here in Norway twice a year calibration would be good enough for me personally.

Timo | October 29, 2012

19" tire diameter is about 703mm. If you have worn 5mm off then track length changes from 2208.5 to 2177 (about). That's about one and half a percent difference, so instead of going 100km/h you are doing 98.5km/h. That's close enough for me, it would still be most accurate speedometer I have ever had in any car I have own.

Brian H | October 29, 2012

Hm, that brings up an idea. With digital tech you could have a "zoomed" sub-speedo that showed the 5 mph bracket you were in, expanded into tenths of a mph, to give at least the illusion of precision!

Volker.Berlin | October 29, 2012

With digital tech you could have [...] (Brian H)

In German we have a saying: "Du könntest Dir auch ein Loch ins Knie bohren und gucken ob Öl kommt." About equally useful.

Captain_Zap | October 29, 2012

Oh what fun it would be if the speedometer were controlled by GPS.

"Sorry officer, there must be a solar storm today."

ltd | October 29, 2012

There is zero need to calibrate this. No other cars do, why should Tesla when its a <1.5% difference.

Has it been an issue in any car ever? I say 'no'...

Think for a moment how any GPS calibration may work and you'll see its problematic. GPS simply is not accurate enough to make any calibration. Remember that when you're driving in a straight line even with 6+ satellites the accuracy is still -/+ 3m typically which is probably a higher margin of error than the circumference difference in a tire.

Brian H | October 29, 2012

More humorlessness? It was an example of excessive precision "for fun".

mrspaghetti | October 29, 2012


A google search leads me to believe gps speed is more accurate than regular speedometers. I've noticed my wife's speedo differs by about 4 mph from gps speed readings on the highway. Could be the difference between a ticket and not.

jkirkebo | October 29, 2012


1. Nobody else does this. It could be another Tesla first. I would really appreciate it. It is a simple software function.

2. You average the difference over several miles, any errors should cancel out. And GPS is more accurate for speed than position anyway. The speed is not derived from distance/time only but also uses doppler shift.

jbunn | October 30, 2012


Your wife's runs 4 mph off at highway speed. Does it vary by speed? Say, is it 2 miles off at half highway speed? Or pretty much off 4 mph at any speed when moving? I'm curious, because over the last 12 years I've come to adjust to the fact that my car seems to run 10% off at any speed as measured by the various radar signs at different speeds I see on the road. Now when I'm on a 40 mph road for example, I lock the cruse on 44.

I think on my car, the spedo cable is "adjusted" by interchangable gears depending on tire and (possibly) rear differential gearing optons.

TV | January 22, 2013

Bottom Line: If the speedometer is GPS based, I'm out. I simply won't buy one, because THAT singular fact would indicate to me that this is a full-blown tracking device. While I am a law-abiding American, I am tired of what has been happening to the FREEDOM and RIGHT TO PRIVACY that the US Constitution promises us. That includes the Second Amendment and all of the rest of it. It shocks me that in as little as 12 years, the politicians and schemers of the world have successfully plotted through "The Patriot Act" and a variety of other abused legislation(s) to take away our Freedom. I am very much PRO TESLA as well as a huge advocate of the science behind Nikolai Tesla, but I am very sure I'm not going to buy one big dang tracking device. Track the non-Americans. Track the Terrorists. But don't water my leg and tell me its raining.

I hope that person is wrong, but I will never participate in that kind of a product. I'll be checking into this further.

HansJ | January 22, 2013

@TV - I assume you don't carry a cell phone then correct?

jjaeger | January 22, 2013

And they constantly stream rearview camera feed (camera on display or not) back to mothership (and big brother). And heaven forbid if any 2nd Amendment articles are placed in the frunk - GPS location and reaview view stream go straight to ATF central as part of their big data repository. All good though - nothing to fear.

Pungoteague_Dave | January 22, 2013

TV, careful there - your post could be used against you in a carry permit hearing. Just sayin, as a carry permit holder in both MD & VA, those kinds of rants can come back to haunt us "right-wingers"

Timo | January 22, 2013

@TV Bottom Line: If the speedometer is GPS based, I'm out. I simply won't buy one, because THAT singular fact would indicate to me that this is a full-blown tracking device.

GPS is not two-way communication. Car doesn't have to tell anyone where it is to get GPS coordinates and use them.

Your cellphone OTOH does have build-in tracking. Also pretty much any credit card, RFID toll systems etc. keep track on where you go and what you are doing. Your web traffic is being monitored, your Google searches are being registered etc. etc. etc.

Not by government (unless you are in some list) but by corporations which use them to target advertisements, spam you and so on.

If you want to avoid that stop using money and any modern communication device, never buy anything and move to nearest forest so that there are no security cameras watching you. In fact you need to move away from society completely, because your friends are being watched and that leads "them" to you.

jat | January 22, 2013

@TV - you do know that GPS is receive-only, right? The GPS receiver picks up broadcast of timestamps from a number of satellites, it knows the orbits of those satellites and can determine where they were at the instant they transmitted the signal, and by calculating the propagation delay from each of them it establishes a 3D position and error bounds (since the propagation speed of radio signals varies with atmospheric conditions). So, the fact that GPS is on in your car doesn't say anything about transmitting your location to anybody.

If you care about that sort of thing, what you should be worried about is your cell phone (and the cellular modem in the car), since its very operation requires that the cellular company have a rough idea of where you are at all times. And certainly don't run Google maps, since it will be sending your location to their servers in order to get the right map tiles.

Aside from that, I would be exceptionally surprised if the speedometer were GPS based as it is a lot more work, uses a lot more power, is unreliable in city canyons due to multipath and lack of line-of-sight to the satellites, etc compared to just measuring how many times the motor turns (which already has to be known to properly control the AC induction motor).

However, I am guessing none of this explanation matters to you because you don't seem the type to let facts get in the way of your righteous outrage.

The Patriot Act is indeed a horrible piece of legislation, but it has nothing to do with why GPSes are in cars.

DouglasR | January 22, 2013


I suspect that TM can indeed determine the location of every car at any point in time, so long as it has an internet connection. We know TM can log into the car remotely, and its GPS coordinates would surely be available along with myriad other data. I'm pretty sure that the speedometer is NOT GPS based.


Just because the speedometer is not GPS based does not mean they are not out to track you.

Timo | January 22, 2013

@DouglasR, as you point it out, you need that internet connection for that to happen. GPS by itself does nothing like that.

jbunn | January 22, 2013

When cell phones went digital, they needed to send packet data so multiple calls can share one radio frequency via time slicing. In order to make sure packets don't collide, phones exchange timing signals from the tower. It's a function of the speed of light, and the frequency of time slicing.

With one tower we know you are in a circle with a hundred yards or so, more or less. With two towers, we have an intersection. Chances are in most places, you'll be in sight of several towers. The network then knows your location to about 50 feet with triangulation, and without GPS.

So TV, it has nothing to do with the car.

We did notice what you had for lunch today, and you might want to consider your salt intake. Also, we noted you did not floss last Tuesday. Finaly, that thing you were considering you told no one about? I wouldn't, if I were you. I'm just giving friendly advice.

Just looking out for you as allways,

Your Big Brother

lolachampcar | January 23, 2013

They have your main spring and they are winding....

stealth_mode | January 23, 2013

Well - i don`t think it is legal to base the speedometer entirely and alone on GPS in a vehicle due to the inaccuracy and other problems associated with GPS-Reception (as mentioned above).
my 2c

Sudre_ | January 23, 2013

One of my, "the government is out to get you" friends got his dog chipped. I jokingly suggested that the government used that to track people because dog lover take their dogs with them everywhere. He returned the dog until they removed the chip.

My wife actually likes that I track her phone. I did it because she is constantly losing it. It doesn't say, "it's under the couch" but at least we know if she lost it in a park, work, home, department store, etc.

hsadler | January 23, 2013

Second part of the question was about tunnel reception. (aside from the notion that speedometer is GPS based)

I worked in the development of GPS and was very surprised, a few years ago, when I was driving a BMW in Switzerland that tracking was good thru their tunnels - one of which was about 35 miles long!!

First part - Using for instantaneous speed measurements in a slow moving vehicle is ludicrous.

An aircraft would be different.

Nominal accuracy of GPS is about half a microsecond - which translates to aporox 3 miles. This is fine tuned with the availability of multiple satellites in different directions. Using for speed measurements would rely on multiple satellites which may not always be visible.

Plus, each one is corrected once per day (or more). If using during that time it would throw you a curve.

We've all seen the occasional jump of the dot showing our location.

Bottom line - why do this when you have a perfectly good system in our cars?

@TV - you're a funny guy !!!