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Vchat20
Landing is the REAL challenge
Premium
join:2003-09-16
Columbus, OH

GPS vs Speedo/ECU disagree / Which one to believe?

So this seems to be a constant from what I can tell. Although I have not tested in a while on other vehicles, previously I have always had luck with the Speedo, ECU, and GPS agreeing +/- 1mph from each other usually being spot on.

With my Focus though I have noticed that while the Speedo and ECU seem to agree, GPS disagrees and usually pits the Speedo/ECU readings a good 3-5mph below GPS readings. This is even with cruise control set. So for example if I am doing 65mph going by the speedo, I am likely closer to 70 if I am to believe GPS readings.

So which one am I to believe here? I have always been one to believe GPS first given the speed calculation there is textbook definition of 'miles per hour' (ie: Takes a location sample one second, takes a new sample the next second, figures distance between the two points and extrapolates out to an hour long timeframe from that second long sample window) compared to a vehicle speed sensor which assumes a given tire size to determine distance travelled.
--
I swear, some people should have pace-makers installed to free up the resources. Breathing and heart beat taxes their whole system, all of their brain cells wasted on life support.-two bit brains, and the second bit is wasted on parity! ~head_spaz


DarkLogix
Texan and Proud
Premium
join:2008-10-23
Baytown, TX
kudos:3
I'd believe the GPS

Also I'd get the android app "torque"

instead of looking back and forth with this app it can display the difference on screen. (very handy, recently I loaded a new tune on my car and was unsure if I needed to correct for tire size or if the tuner had already (e-mail tune ya know)

so I opened the app plugged in the OBD2 to bluetooth and put the GPS vs ecu display up then got to a fairly consistent speed and they agreed with in less than 1mph.
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Vchat20
Landing is the REAL challenge
Premium
join:2003-09-16
Columbus, OH
Actually I already use Torque and know about the speed difference gauge which usually shows, as noted, around 3-5mph difference.

What got me was in other cases at worst the GPS and ECU agreed but the speedo was around ~5mph OVER (newer cars, I assume a further nanny feature so they think they are doing 5mph more than they really are). In this case the ECU and Speedo agree but are reading UNDER what the GPS reading is giving me.

Usually this kind of thing breaks down to tire size differences from stock, but on this car there isn't even much room to go a different size than stock and I am pretty sure it is stock so that is out of the question.

Was mostly curious though as I've made it a mental note to assume that ~3mph difference when watching the speedo.
--
I swear, some people should have pace-makers installed to free up the resources. Breathing and heart beat taxes their whole system, all of their brain cells wasted on life support.-two bit brains, and the second bit is wasted on parity! ~head_spaz


DarkLogix
Texan and Proud
Premium
join:2008-10-23
Baytown, TX
kudos:3
Well if the ECU and the GPS agree then I'd go with it being a faulty spedo

If it were tire size the difference would be progressive but it sounds linear like the needle is just been pushed out of line

IE if its tire size or gearing then the difference would be greater at say 90mph than at 10mph (when doing the test with torque I'd get to a high speed and set cruise control) if the ECU and GPS agree at say 70 then its not tires or gearing.


Vchat20
Landing is the REAL challenge
Premium
join:2003-09-16
Columbus, OH
Sorry, mistyped on that one. By 'other cases' with the ECU and GPS agreeing I meant other cars and not particularly this one. Don't know why it didn't go from my head to my fingers. :P

But yeah, I appreciate the responses. I'm pretty much just going to go by the mental reminder to add 3mph to what the speedo reads even if just to play it safe and keep my own sanity in check.
--
I swear, some people should have pace-makers installed to free up the resources. Breathing and heart beat taxes their whole system, all of their brain cells wasted on life support.-two bit brains, and the second bit is wasted on parity! ~head_spaz


Doctor Olds
I Need A Remedy For What's Ailing Me.
Premium,VIP
join:2001-04-19
1970 442 W30
kudos:18

1 edit
reply to Vchat20
Verify that the tires are the correct size for your vehicle.

You can always have your ride checked and calibrated if not accurate. Otherwise you risk future speeding tickets if you or others are driving the vehicle and are relying on the speedometer if it reports 3 MPH to 5 MPH slower than true speed.

»www.yelp.com/biz/evans-speedomet···columbus
Evans Speedometer Calibration Service

Categories: Auto Repair, Towing
3440 Morse Rd
Columbus, OH 43231
Neighborhood: Northland
(614) 471-7535
--
What’s the point of owning a supercar if you can’t scare yourself stupid from time to time?


saillaw
Premium
join:2007-05-08
Dismay
kudos:2
reply to Vchat20
I've checked a dozen or so new vehicles with stock tires comparing GPS speed to speedo speed. In every single case the speedo speed read faster than the GPS speed. (Meaning for example that if the speedo read 65, then the GPS read 61-63.)

I can't help but wonder if this is intentional on the part of the manufacturers. There are three reasons why having the speedo read artificially fast is good for the manufacturer. First, for liability reasons, it is better for them for you to to be going slower than you think you are, rather than faster. Second, if your speedo is off by reading 5-10% faster than you are really going, that means you will run through your warranty 5-10% faster than your real distance traveled. Third, having your speedo read fast by 5-10% makes your fuel economy appear 5-10% better than it really is (assuming you calculate based on your odometer which is driven by your speedo).
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Badonkadonk
Premium
join:2000-12-17
Naperville, IL
kudos:5
Reviews:
·Dish Network
We have a 2009 MB R350 and it does the same thing. The speedo reads 2.5 mph faster than the actual (GPS) speed. My 2010 Mazda 3 doesn't do this. We have a 2013 Subaru Impreza too, but I haven't tested it out yet. I will.

Sounds like a class-action suit ready to happen if the mileage and warranty issues are true. I'll have to measure the actual distance and see whether it's affected. I have a 220 mile trip coming up tomorrow and I'm going to check it out. If it truly is throwing off the odometer measurement then I'm going to talk to Mercedes about it.


saillaw
Premium
join:2007-05-08
Dismay
kudos:2
Out of curiosity I just sent a short email to a friend who is a class action lawyer. He said there have been several large class action settlements on this already, he seemed to recall one for Honda and one for Nissan. So I guess you probably missed the boat on this already.

Badonkadonk
Premium
join:2000-12-17
Naperville, IL
kudos:5
Interesting. Well, even so, it can't hurt to bitch about this if it's really messing up the mileage count.


saillaw
Premium
join:2007-05-08
Dismay
kudos:2
reply to Badonkadonk
Also you say your MB is a flat 2.5mph difference, at any speed? My 2010 Subaru is off by 5% (not a flat amount), my 2012 Toyota is 8.5%, and my 2013 Toyota is 7%.

Badonkadonk
Premium
join:2000-12-17
Naperville, IL
kudos:5
Reviews:
·Dish Network
Well, that's a good point. I notice it on the highway when I put the car in cruise. In city driving is so irregular, that I haven't paid much attention. I will tomorrow and see how it goes. I would imagine that it's probably a percentage as you're saying.


DarkLogix
Texan and Proud
Premium
join:2008-10-23
Baytown, TX
kudos:3
Well via my torque app my GPS va ECU are with in less than 1mph diff (on a smooth strait road, bumps throw the gps off a little)

any inaccuracy of the speedo I'd attribute to it being a dial and looking at it from an angle that's is apparently wrong

if the ECU knows the real speed and is right then the odometer will have the correct mileage.

one reason I like digital over dial
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saillaw
Premium
join:2007-05-08
Dismay
kudos:2
For most cars (might be different now with cars that have built in GPS) the ECU gets its speed from the same transponder as the speedo. So there should be no difference between the two.


DarkLogix
Texan and Proud
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join:2008-10-23
Baytown, TX
kudos:3
said by saillaw:

For most cars (might be different now with cars that have built in GPS) the ECU gets its speed from the same transponder as the speedo. So there should be no difference between the two.

correct shouldn't be short of a dial issue.

I know on my car it reads the speed from the ABS sensors on the rear wheels and the speedo is controlled by the ECU
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»www.change.org/petitions/create-···imcity-4


C0deZer0
Oc'D To Rhythm And Police
Premium
join:2001-10-03
Tempe, AZ
reply to Vchat20
It's pretty well known that the speedo in most cars usually is about 4% off from your actual speed, which can only really be measured accurately with a telemetry meter. That said, a GPS unit with a good, strong signal can easily be more accurate than the standard speedometer.

Cracked.com even had an article about "# pieces of technology we trust (that lie to us)" and they brought this up. The Scary thing is that the speedos in cars that only cost $15k or less actually were more accurate on average than your typical luxury car.

Further, any electrical issues could also push it out of wack, to as much as 10% inaccuracy.

The thing is, this also applies to the speedo in a police officer's vehicle, and naturally, if that's inaccurate, it can and likely may be measured/used against you in terms of a speeding violation. While it is possible to beat such a rap if you have records of doing a recalibration on the speedometer, that's usually almost as expensive to do as it would be to deal with the ticket (+ traffic school).
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DarkLogix
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join:2008-10-23
Baytown, TX
kudos:3
would using a SCT X3 to recalibrate it for my tires and then comparing it to gps and being less than 1 mph diff (generally when hitting a bump was the largest diff) count?

though I doubt they'd buy that I was only doing 60 and no 100.
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»www.change.org/petitions/create-···imcity-4


C0deZer0
Oc'D To Rhythm And Police
Premium
join:2001-10-03
Tempe, AZ
You'd have to provide/submit records showing that your speedo was sucha-sucha percentage "inaccurate" (e.g. the before) compared to what the recalibrated accuracy is (after).

IF it can provide something printable, it MAY hold up in court; but I'm no lawyer, so feel free to take that advice with the requisite grains of salt. Of course, it has to stand to reason that the variance of inaccuracy is enough to have made you think you were "legal" when you really weren't.
--
Because, f*ck Sony


Anonymous_
Anonymous
Premium
join:2004-06-21
127.0.0.1
kudos:2
Reviews:
·Time Warner Cable
said by C0deZer0:

You'd have to provide/submit records showing that your speedo was sucha-sucha percentage "inaccurate" (e.g. the before) compared to what the recalibrated accuracy is (after).

IF it can provide something printable, it MAY hold up in court; but I'm no lawyer, so feel free to take that advice with the requisite grains of salt. Of course, it has to stand to reason that the variance of inaccuracy is enough to have made you think you were "legal" when you really weren't.

Sorry but if a cop gives you a ticket for going 2-3mph over the limit he is just a jack***...
--
Live Free or Die Hard...


C0deZer0
Oc'D To Rhythm And Police
Premium
join:2001-10-03
Tempe, AZ
said by Anonymous_:

Sorry but if a cop gives you a ticket for going 2-3mph over the limit he is just a jack***...

All the more reason to stick it to him by doing such a service and verifying that you were "legit" and he wasn't.
--
Because, f*ck Sony


DarkLogix
Texan and Proud
Premium
join:2008-10-23
Baytown, TX
kudos:3
Well if you were 3 off and he was 3 off in the other direction then that could be 6mph diff

But ya if he pulled you over for 6mpf then he's a ********


Gomez
ha ha, charade you are
Premium,Ex-Mod 06-11
join:2001-02-21
Atlanta, GA
reply to saillaw
Correct. The Speedo and the ECU are calibrated based on the tire info on the door.. It's the best they can do.. Unlike aircraft, they aren't measuring airspeed, which has it's own set of problems.

A decent GPS with a good sampling algorithm is going to be more accurate.

In GA 0-14 over limit is 0 points, so generally don't worry about it in ATL metro.. There are certain areas where I'm a bit more cautious ( thus the detector ).

6 over generally won't raise an eyebrow around here.. Been tagged with laser at 14 over, with nothing. Only exception would a school zone, which is usually zero tolerance.. I'm ok with that.

If someone got ticketed an highway speeds for 6 over, there has to be more to the picture..

-G
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DarkLogix
Texan and Proud
Premium
join:2008-10-23
Baytown, TX
kudos:3
said by Gomez:

Correct. The Speedo and the ECU are calibrated based on the tire info on the door.

Not 100% right

From the factory they are but depending on the owner that can be changed.

so its more right to say based on the tire size that's set in the ECU
In an older car you couldn't adjust it so much, and the actual number used is the tire manufacturer's revolutions per mile, which is slightly more accurate than using the tire size info for that calculation.
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»www.change.org/petitions/create-···imcity-4


Gomez
ha ha, charade you are
Premium,Ex-Mod 06-11
join:2001-02-21
Atlanta, GA
Sure.. Main point being is that any calculation the car can make is based on assumptions. If the assumptions are flawed, the calculation is flawed.

-G


Doctor Olds
I Need A Remedy For What's Ailing Me.
Premium,VIP
join:2001-04-19
1970 442 W30
kudos:18
reply to DarkLogix
said by DarkLogix:

Well if you were 3 off and he was 3 off in the other direction then that could be 6mph diff

But would it get you ticketed?

Off by 3 so 47 instead of 50 (posted Limit) and 3 in the other direction 53

Off by 3 so 52 instead of 55 (posted Limit) and 3 in the other direction 58.
--
What’s the point of owning a supercar if you can’t scare yourself stupid from time to time?


DarkLogix
Texan and Proud
Premium
join:2008-10-23
Baytown, TX
kudos:3
said by Doctor Olds:

said by DarkLogix:

Well if you were 3 off and he was 3 off in the other direction then that could be 6mph diff

But would it get you ticketed?

Off by 3 so 47 instead of 50 (posted Limit) and 3 in the other direction 53

Off by 3 so 52 instead of 55 (posted Limit) and 3 in the other direction 58.

well only if the cop is a jerk.

and if say yours is reading slow say showing 50 when you're doing 53 and the cops is fast reading 56 when you're 53 then thats 6mph over.

but still the cop would have to have it out for you for a 6mph over stop.
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Kearnstd
Space Elf
Premium
join:2002-01-22
Mullica Hill, NJ
kudos:1
reply to Vchat20
This is likely why many cops allow for a certain over the limit. Their radar or lidar is precise but the speedo never will be.

Interesting would be GPS vs the cops, are the satellites or the laser gun more accurate. I do know when testing for speed records these days that they tend to use a combination of GPS and laser speed guns. So on board sensing is not terribly accurate.

An exception might be police motorcycles, I saw on Modern Marvels once I think it was that motorcycle units get their speedos calibrated more than once a week, allowing them to tail a car and get its speed.


saillaw
Premium
join:2007-05-08
Dismay
kudos:2
reply to Vchat20
Where I am currently working (Saudi Arabia) they have a lot of those signs that show you your speed and blink if you are speeding. I've noticed that with one of my cars the speed that those signs show is very sporadic, it will show the correct speed -- suddenly jump to 20kph faster -- suddenly jump 20kph slower, etc. I'm not sure if those signs use laser or radar, but I assume radar.

Anyway, it makes me wonder if radar guns are subject to the same sorts of errors? The car that causes this is very boxy with many flat surfaces, the more curvy car does not cause these jumps (which I would have expected the opposite). This is consistent with lots of these signs (i.e. I am not just talking about one sign at one location).
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DarkLogix
Texan and Proud
Premium
join:2008-10-23
Baytown, TX
kudos:3
get a B-2 shaped car.