# GPS Based Speedometer and Odometer



## obermd (Mar 3, 2012)

On a whim today I brought up the Torque Lite Speedometers to compare them to my car's speedometer. Torque Lite had both GPS and ODBII based speedometers. It has a GPS based trip odometer. What I discovered is that the Cruze's speedometer and odometer appear to be GPS based and not wheel spin based. If this is true, then the entire issue of speedometer error in the Cruze goes away. Confirmation of this would also mean that the DIC's MPG errors are entirely a result of the way GM estimates fuel used.


Obviously, if the car can't get a GPS lock it would have to fall back to wheel spin for measuring speed and distance.

Question - can other CT members confirm/non-confirm what I saw today?

Thanks.


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## TechCruzer (Mar 15, 2012)

obermd said:


> What I discovered is that the Cruze's speedometer and odometer appear to be GPS based and not wheel spin based Obviously, if the car can't get a GPS lock it would have to fall back to wheel spin for measuring speed and distance.
> 
> Question - can other CT members confirm/non-confirm what I saw today?
> 
> Thanks.


:question: Not trying to make light, but how did you come to the idea those two things are GPS based? Android phones do have accelerometers in them & I think that is what should be providing the app it's information.


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## obermd (Mar 3, 2012)

TechCruzer said:


> :question: Not trying to make light, but how did you come to the idea those two things are GPS based? Android phones do have accelerometers in them & I think that is what should be providing the app it's information.


Fair question. Torque Lite has two speedometers, labeled "Speed (GPS)" and "Speed (ODB)". It also has a "Trip Distance". Assuming these are labeled accurately the "Speed (GPS)" is has to be using the GPS from my phone. The "Speed (ODB)" would be coming from the car. I'm not sure how the "Trip Distance" is computed. What I noticed was that the "Speed (GPS)" more closely matched the DIC's digital speed readout than the "Speed (ODB)" did, especially during acceleration and deceleration. In fact the "Speed (ODB)" jumped above and below both the DIC speedometer and "Speed (GPS)" during some accelerations.


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## TechCruzer (Mar 15, 2012)

obermd said:


> Fair question. Torque Lite has two speedometers, labeled "Speed (GPS)" and "Speed (ODB)". It also has a "Trip Distance". Assuming these are labeled accurately the "Speed (GPS)" is has to be using the GPS from my phone. The "Speed (ODB)" would be coming from the car. I'm not sure how the "Trip Distance" is computed. What I noticed was that the "Speed (GPS)" more closely matched the DIC's digital speed readout than the "Speed (ODB)" did, especially during acceleration and deceleration. In fact the "Speed (ODB)" jumped above and below both the DIC speedometer and "Speed (GPS)" during some accelerations.


So you are not connecting an OBDII to your DLC to test, but rather only using the GPS function?


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## obermd (Mar 3, 2012)

TechCruzer said:


> So you are not connecting an OBDII to your DLC to test, but rather only using the GPS function?


I'm running both at the same time. The ODBII interface is a bluetooth interface and my phone is connected to it to receive the ODB information and at the same time using its internal GPS to compute the GPS speed.


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## jblackburn (Apr 14, 2012)

GPS signals just aren't good enough to base a car's speedo entirely off of. For instance, a tunnel or something. Your speedometer doesn't stop working when you go into a tunnel.

Perhaps the in-dash display lags behind by a second or two.


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## TechCruzer (Mar 15, 2012)

obermd said:


> I'm running both at the same time. The ODBII interface is a bluetooth interface and my phone is connected to it to receive the ODB information and at the same time using its internal GPS to compute the GPS speed.


Ok... I'll try it out tomorrow. I have the full app & I'll see what happens with & without the GPS enabled on my Android.


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## obermd (Mar 3, 2012)

jblackburn said:


> GPS signals just aren't good enough to base a car's speedo entirely off of. For instance, a tunnel or something. Your speedometer doesn't stop working when you go into a tunnel.
> 
> Perhaps the in-dash display lags behind by a second or two.


In my original post I did note that you can't always use the GPS signal for speed and the car would have to use its internal measurements such as wheel spin, engine RPM, and gear when the GPS is unavailable. However, when running on level ground using cruise control the DIC's digital speed readout matched the Speed (GPS) and not the Speed (ODB) being reported. In open areas my phone will usually acquire 8 or more satellites. The Cruze has a GPS receiver which is used for the onboard compass. Why not use it for other purposes as well?

It's because of the chance of losing GPS signal that I asked for other CT members to see if they could confirm or refute my apparent findings using their ODB interfaces and GPS systems. I was expecting the ODB speed to match the speedometer, not the GPS speed.


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## jblackburn (Apr 14, 2012)

obermd said:


> In my original post I did note that you can't always use the GPS signal for speed and the car would have to use its internal measurements such as wheel spin, engine RPM, and gear when the GPS is unavailable. However, when running on level ground using cruise control the DIC's digital speed readout matched the Speed (GPS) and not the Speed (ODB) being reported. In open areas my phone will usually acquire 8 or more satellites. The Cruze has a GPS receiver which is used for the onboard compass. Why not use it for other purposes as well?
> 
> It's because of the chance of losing GPS signal that I asked for other CT members to see if they could confirm or refute my apparent findings using their ODB interfaces and GPS systems. I was expecting the ODB speed to match the speedometer, not the GPS speed.


Makes sense. Wonder what it would do up on jack stands with a clear view of the sky.


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## Swampassjr (Mar 25, 2012)

are you guys saying that the Cruze's speedometer is based off of GPS? That sounds far fetched


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## gman19 (Apr 5, 2011)

jblackburn said:


> Makes sense. Wonder what it would do up on jack stands with a clear view of the sky.


^^! Beat me to it...this was what I was thinking down the entire thread! Someone's gotta try this, could prove interesting....


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## obermd (Mar 3, 2012)

Swampassjr said:


> are you guys saying that the Cruze's speedometer is based off of GPS? That sounds far fetched


Why? Newer commercial aircraft use GPS for everything, including speed. As long as there is a reasonably close and consistent backup system using GPS for your speedometer eliminates the issue of speedometer error. The Cruze does have a built in GPS receiver and it uses it for the compass display. That said, I was surprised by what I saw yesterday, which is why I started this thread.


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## EcoCruzer (Mar 4, 2011)

When I had my snow tires on the car (16" rims), my speedometer was out 2 KPH. With the stock ECO wheels (17"), speedometer is right on with my Garmin Nuvi GPS. This was also the case comparing my Garmin to Torque on my phone.


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## Swampassjr (Mar 25, 2012)

obermd said:


> Why? Newer commercial aircraft use GPS for everything, including speed. As long as there is a reasonably close and consistent backup system using GPS for your speedometer eliminates the issue of speedometer error. The Cruze does have a built in GPS receiver and it uses it for the compass display. That said, I was surprised by what I saw yesterday, which is why I started this thread.


"aircraft" don't have to worry about lost GPS signal and other issues like our cars will. You provide the proof of this, and then i'll believe it


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## rbtec (Feb 3, 2012)

Calling BS.


Sent from my Autoguide iPhone app


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## obermd (Mar 3, 2012)

Swampassjr said:


> "aircraft" don't have to worry about lost GPS signal and other issues like our cars will. You provide the proof of this, and then i'll believe it


For the Cruze - page 5-4 of the 2012 owners manual:

*Compass

*The vehicle may have a compass display in the Driver Information Center (DIC). The compass receives its heading and other information from the Global Positioning System (GPS) antenna, StabiliTrak, and vehicle speed information. Avoid covering the GPS antenna, located on the roof, for long periods of time with objects that may interfere with the antenna's ability to receive a satellite signal. The compass system is designed to operate for a certain number of miles or degrees of turn before needing a signal from the GPS satellites. When the compass display shows CAL, drive the vehicle for a short distance in anopen area where it can receive aGPS signal. The compass system will automatically determine when the GPS signal is restored and provide a heading again. See _Compass Messages on page 5‑28 _for more information on the messages that may be displayed for the compass.

The only messages listed on 5-28 are "CAL" for calibration and "--" for needs service.

Obviously a GPS enabled car would need to support non-GPS based speed reporting for when it loses GPS lock. I'm kind of surprised at using GPS for StabiliTrak as that could very easily be done with accelerometers. I know the 2013 Malibu uses accelerometers as there is a recall for a software bug reading them.

================
For commercial aircraft: See the FAA's web-site at Navigation Services - Global Navigation Satellite System. In addition, Boeing has been retrofitting older aircraft and building directly into their newer aircraft GPS enabled autopilots. I don't know about Airbus, but I would be very surprised if they weren't doing the same.


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## Swampassjr (Mar 25, 2012)

obermd said:


> For the Cruze - page 5-4 of the 2012 owners manual:
> 
> *Compass
> 
> *The vehicle may have a compass display in the Driver Information Center (DIC). The compass* receives its heading and other information* from the Global Positioning System (GPS) antenna, StabiliTrak, and vehicle speed information. Avoid covering the GPS antenna, located on the roof, for long periods of time with objects that may interfere with the antenna's ability to receive a satellite signal. The compass system is designed to operate for a certain number of miles or degrees of turn before needing a signal from the GPS satellites. When the compass display shows CAL, drive the vehicle for a short distance in anopen area where it can receive aGPS signal. The compass system will automatically determine when the GPS signal is restored and provide a heading again. See _Compass Messages on page 5‑28 _for more information on the messages that may be displayed for the compass.




All this is saying, is the compass uses those 3 inputs to determine which direction you are heading. That does not say all these systems use the GPS


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## obermd (Mar 3, 2012)

Swampassjr said:


> [/FONT][/FONT][/SIZE]All this is saying, is the compass uses those 3 inputs to determine which direction you are heading. That does not say all these systems use the GPS


It also doesn't say that the GPS is not used for other purposes. So we're back to my original question - can anyone confirm/refute with live data monitoring what Torque Lite appears to have shown me yesterday? The answer right now is a definite "maybe". I was actually expecting my the DIC's speedometer to match the ODB speed in Torque Lite, not the GPS speed.


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## landrystephane92 (Jan 3, 2012)

First gear, gas pedal floored, hand brake engaged, release clutch.

If you see speed on your gauge, it's not GPS based.

(you could also just jack the car on one side)

I know for a fact it's not because of one of my threads. My speedo was going crazy due to a bad connector on the tranny.


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## obermd (Mar 3, 2012)

landrystephane92 said:


> First gear, gas pedal floored, hand brake engaged, release clutch.
> 
> If you see speed on your gauge, it's not GPS based.
> 
> ...


Thank you. Finally someone who understood that I was simply asking for confirmation one way or the other.


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