PDA

View Full Version : Digital and analog speedos don't match



stratofortress
06-05-2013, 05:38 PM
So with the digi speedo up it never quite matches the analog speedo. One is about 3-5mph off around 65 and I can't for the life of me remember which one reads faster so I'll update after driving home from work.

It's not a huge issue but it is an annoyance; just seeing if anyone else has this problem and if a simple recalibration is the fix or the dealer is gonna have to pull the dash.

F1AT
06-05-2013, 06:24 PM
So with the digi speedo up it never quite matches the analog speedo. One is about 3-5mph off around 65 and I can't for the life of me remember which one reads faster so I'll update after driving home from work.

It's not a huge issue but it is an annoyance; just seeing if anyone else has this problem and if a simple recalibration is the fix or the dealer is gonna have to pull the dash.

Also noticed this. Not sure what a fix would be.
Let me know if you do figure anything out! :)

jguerdat
06-05-2013, 06:27 PM
No problem here. I wonder what happens if the car was spec'ed for 16" wheels and you're running 17", or the reverse...

Leotxn
06-05-2013, 06:48 PM
I found with 16" stocks, 2012 Abarth, my digital reading was 3mph under what actual speed is. With 17" wheels the digital is dead on correct (matches GPS unit speed and roadside radar units). Also needle reading doesn't match digital but the digital I find is dead on now.

stratofortress
06-05-2013, 07:10 PM
No problem here. I wonder what happens if the car was spec'ed for 16" wheels and you're running 17", or the reverse...I have the 17s...but I thought overall diameter was maintained? If not...that's pretty dumb of FIAT.


I found with 16" stocks, 2012 Abarth, my digital reading was 3mph under what actual speed is. With 17" wheels the digital is dead on correct (matches GPS unit speed and roadside radar units). Also needle reading doesn't match digital but the digital I find is dead on now.Still doesn't make sense that the two won't match -- only seems logical that they would both get data from wheel speed sensors. If the digi and analog are being fed data from different output sources that'd be another dumb redundancy of engineering.

I'm thinking the needle just needs to be reset -- from what I recall the digi felt right and the analog always felt like it was lying to me.

So now that this is an issue across a few cars...anyone wanna chime in with a fix? I'm sure someone's had this corrected.

wilbmeister
06-05-2013, 10:38 PM
My seems to be right on. Both the analog and digital match and they both match the speed that shows on my Garmin. I guess I'm lucky that mine all match up and are accurate.

jguerdat
06-06-2013, 07:49 AM
I have the 17s...but I thought overall diameter was maintained? If not...that's pretty dumb of FIAT.

Diameter is about 1" different. 22.9" for the 16s, 23.7" for the 17s.

jguerdat
06-06-2013, 07:56 AM
My seems to be right on. Both the analog and digital match and they both match the speed that shows on my Garmin. I guess I'm lucky that mine all match up and are accurate.

Mine match, too. No idea why there would be a discrepancy unless it's the whole diameter thing which could only be solved by using the proper sizes. The difference in diameter is about 3.4% or 2 MPH error at 60, 2.4 MPH at 70.

I suppose there is no change in the speedometer setup with the different wheels from the factory but unless we know how the two reading are calculated it's all guesswork. The needle may be based on the transmission and the digital based on ABS readings. Who knows?

Mr. Man
06-06-2013, 08:35 AM
My analog has never been accurate, and I've had the 16s and 17s. It just moves too slowly, which is sad because I thought the gauges were really cool when I first saw this car

Venom51
06-06-2013, 10:40 AM
Mine (in my Sport) have always been off by about 1-2 mph.

Ryephile
06-06-2013, 10:44 AM
If you think the needle vs. digital readout don't match, try reading the OBDII output, LOL. FWIW, the OBDII output is likely the most accurate. With factory 17" setup a dashboard digital readout indication of 75 MPH resulted in an OBDII read of 73 MPH.

I didn't do a GPS speed to check.

Robert Nixon
06-06-2013, 11:07 AM
mine probably don't match, I only look at the digital read out since the steering wheel blocks just about all the speedometer anyway. Not a big deal to me. At least if anyone gets a ticket for being 3 over you have something to discuss in court!

hownowcb
06-06-2013, 11:45 AM
I've run 16" wheels and tires in the winter, and now my stock 17" wheels and tires, and with both sets, my GPS confirms my analog speedometer is 99.9% accurate. It's a little embarrassing, but I only learned a few months ago how to have the digital speed show on my display. Since I've read for ages about how others' digital and analog displays have disagreed, I've noted with keen interest how mine have been behaving, so here's my two cents worth:

Because I know my analog display is accurate (verified by GPS), I tend to rely on it more than I do the digital. Just recently, I observed that when accelerating, briefly, my digital speed was at least 3-4 mph slower than my analog gauge displayed. Moments later, at a steady speed, they were pretty much in agreement. Based on another observation of the behavior of the digital display in general, I think there is a considerable lag time between what speed you might be traveling, and when the digital display actually is capable of showing that on the dash. And this results in a perceived "display disagreement", which in fact may only be momentary.

I base my reasoning on observing the digital temperature display with regularity (due to the horrible climate I live in) -- when my car is parked inside the garage, it often takes two to three miles of driving at roughly 30 mph for the temperature readout to display the obvious to me, large difference between the inside the garage and outside temperatures. I think there's just an unusually long delay between when an event is happening, and when the readout is going to display it.

One could logically remark that this is not an ideal circumstance, moreso in regard to speed readout. On the other hand, unlike some vehicles, our Fiats aren't limited to a single type of speed display. Bottom line for me is, though the digital readout is convenient, it's much safer to rely on the analog display, especially in rapidly changing speed situations. In steady-state cruising situations I'm just fine trusting the digital display.

Bottom line: I don't think that technically there's a defect in anybody's displays. Is the design execution flawed? Probably, but not so much as to be a liability in anything but the most rare of circumstances. I don't worry about it, but I'm also not overly burdened by OCD. Some people are, and it compels them to post about things like this.

Vaejovis carolinanus aka lowconabarth
06-06-2013, 12:53 PM
I am running 16s and the speedo seems accurate for both dig and analog as far as I can tell. The police have those devices that post your speed and the limit. That is how I measured mine.

dylansi
06-06-2013, 01:27 PM
My analog is always around 2mph faster than the digital. Running the stock 16"s

Leotxn
06-07-2013, 05:20 AM
I compare mine with gps unit while highway driving & every morning when driving on base as they have radar at gates showing speed of approaching vehicles. Not a huge issue to me I just know it was off with 16" compared to running 17" now.

DuckDodgers
06-08-2013, 12:19 AM
Rough figures show that the 16" tires should show about .9 mph slower per 25 mph. Putting it simply; at 75 real mph, the car with 16" tires will show about 72 mph.

The P7's are 22.9" diameter while the Zero's ar 23.7". The math makes the 16" 881.2 revs per mile and the 17" 851.4 revs per mile. That's a 3.5% difference...which is over the commonly accepted 3% rule.

My guess would be a programing error at build time. With a 3+% difference, I'd expect the input to be adjustable with the Fiat computer system.

jguerdat
06-08-2013, 07:58 AM
FWIW, my phone GPS had me within a 1 MPH error compared to the digital readout on 17" wheels with Dunlop Z2 215/40-17 tires which are supposedly 0.1" taller than stock. Depended on the exact speed/time/place as to whether there was an error. The analog seemed slightly low (kinda like -.5 MPH)...

13VaAbarth
06-08-2013, 05:28 PM
My digital and analog are off as well I say 2 to 3mph and I have the 17's if I am on the hwy I just cruise to what the digital is saying since it is the lower speed and I feel that it is more accurate but I might have to test it on gps somehow.

DS Ocampo
06-09-2013, 11:42 PM
Thought I was alone on this lol .. I have stock 16s and they are off by a few mph .. Ok now when I plug my Tom Tom in the dash the speedo is off by 3 from digital and digital is off by 1 from tomtom .. Lol .. I quit caring the day I get pulled over that cop is gonna loose his mind

Cop: do you know why I pulled you over?
Me: no
Cop: do you have any idea how fast you were going back there ?
Me: o-O .. Well sir ... I got 75 on the speedo 78 on the digital read and 77 on my GPS .. So I really have no clue.... Wait what did you clock me at?
Cop: 80
Me: o-0 ......???????

stratofortress
06-10-2013, 12:26 PM
So yeah...analog is too fast vs. the digital. And this was at a constant speed so there's no possibility of it being a lag issue (besides, the analog shouldn't lag anyways).

I don't think this is really being OCD...seems pretty reasonable for me to expect things to work perfectly in the second model year of a modern car. OCD is wishing the digital font was Calibri (because Calibri is the best font ever).

That said...I'll have the dealer check it out when I'm at my first oil change; I'll try to remember to post an update.

ukeluthier
06-11-2013, 11:41 AM
Hate to disappoint all you OCD folks, but as a retired laboratory engineer I have to inform you that the chances of two measuring devices agreeing exactly are miniscule, especially if different measuring technologies are involved. In the science of metrology (measuring) there exists a universal concept known as tolerance. No measuring device is perfectly accurate or precise (two different things, by the way). All are manufactured to a specification of some tolerance... an allowable inaccuracy usually expressed in terms of +/- a certain percentage of the reading.

Now I've never looked behind the dash of my Abarth, so I don't know if the analog speedometer is driven in the old-fashioned way off a mechanical cable spinning a magnet inside a pivoting metallic cup with the needle attached and relying on eddy currents inside the cup to drive the needle against a watchspring, or whether the analog gauge is simply an analog voltmeter displaying the electronic integration of the pulses from a pulse counter, but in either case the accuracy will be highly dependent on the linearity and initial pre-loading of the tension on the watchspring. I'm sure the digital speedo is counting pulses from a sensor mounted either at the wheel or at the instrument cluster end of the same cable that may be driving the analog speedometer and displaying the speed equivalent of a given number of pulses occurring over an interval, a process that is subject to something called aliasing error.

Regardless, the technologies involved for each display are different, and each have their own allowable tolerances and causes of error. The chances of them agreeing precisely under any given circumstance are coincidental. That's no fault of Fiat... it's a "fact of life" of measurement. Careful calibration against laboratory standards could make the agreement better, but it wouldn't totally eliminate it.

stratofortress
06-11-2013, 12:18 PM
That's all good and well except some people are reporting no differences, and some are.

My previous car was perfect between the displays...and that was 10 years older than my Fiat! Also...1-2mph is a reasonable tolerance. Nearing 4-5mph discrepancy seems like a pretty big difference and at 65mph that's over 5% (which I know statistically, is pretty bad). I don't want perfect or precise but having to add 2-5mph on a progressive rate depending on my speed is lame. If the digi was off that wouldn't bother me...but the analog is there to stay and is the main gauge on the cluster so I should be able to trust it.

So please stop referring to this as OCD; I'm obsessive when I detail my car. Mechanical stuff...as long as it works I'm happy. This is a brand new car and everything should work....especially the thing that tells me how fast I'm going. Period. All I wanted to know was if my issue was a one off, and apparently it isn't.

BigDaddySRT
06-11-2013, 12:22 PM
The Digital and Analog Speedometer receive the same electronic signal.

ukeluthier
06-11-2013, 12:49 PM
That's all good and well except some people are reporting no differences, and some are.



That's obviously because they are all built to fall within an allowable tolerance... some read a bit high, some a bit low. If the analog and digital meters on a given vehicle are near the limits of their tolerance in the opposite directions, then the agreement will be lousy. Nothing mysterious about that. If the error is in the same direction, then they may agree well -- even if the absolute reading is not very accurate.

According to Wikipedia, EU international agreements dictate:

- The indicated speed must never be less than the actual speed, i.e. it should not be possible to inadvertently speed because of an incorrect speedometer reading.
- The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. For example, at 80 km/h, the indicated speed must be no more than 92 km/h.

There is apparently no US standard for speedo accuracy for non-commercial vehicles. Commercial vehicles must read within +/- 5 mph at 50 mph.

Manufacturers build to a price point. Chances are that precludes the expense of the labor involved in individually calibrating speedometers against a laboratory standard unless you are buying a Rolls or Aston Martin. As long as the speedometers read within the allowable legal standards, manufacturers of inexpensive (read Fiat) vehicles aren't going to worry about it.

If you are one of the unlucky ones who got a car whose speedometer displays happen by chance to read at the opposite ends of their tolerances, you may feel justified in being upset... but don't expect Fiat to be legally obligated to make them agree. An individual dealer may swap yours out just to maintain customer good will -- and Fiat might actually back it up for the same reason -- but they don't have any other real obligation.

But remember... just because the displays agree doesn't mean they are accurate. They can simply be reading 5% off in the same direction.

ukeluthier
06-11-2013, 12:58 PM
The Digital and Analog Speedometer receive the same electronic signal.

That may indeed be true, but the display technologies are different -- as I said above -- and each type of display is subject to its own set of sources of error. Any analog meter display will depend on the consistency of the tension and linearity of the tension of the spring, and any digital display will have aliasing and other errors, even if both displays are fed precisely the same train of electrical pulses.

stratofortress
06-11-2013, 01:01 PM
Understood and makes sense (engineers tend to have that ability) :)

Still...I'll mention it to my studio and see if a simple recalibration might fix the issue.