[vina]

Originally Posted by Mustang.101

You can't rely on GPS speed as GPS does not take into account the gradient of the road. But can somebody, who has more knowledge about GPS, confirm this.

For instantaneous speed, not just gradient of the road but also GPS's own inaccuracy will also come into picture. I don't know whether the particular GPS in question does any averaging - but if it does (e.g. over a km) then that would be accurate. However that is not something to be compared to a speedo (unless you could maintain constant speed)

Best is to check the milestones (or even your odometer) and use your watch - both are pretty accurate.

[shivmarwaha]

I too noticed the same in my figo. There is clearly a ten percent higher reading in the speedometer. Does thhis also mean that the odometer is also ten percent fast. Which means that the fuel efficiency that we get has to be corrected downwards by ten percent.

[vina]

Originally Posted by shivmarwaha

I too noticed the same in my figo. There is clearly a ten percent higher reading in the speedometer. Does thhis also mean that the odometer is also ten percent fast. Which means that the fuel efficiency that we get has to be corrected downwards by ten percent.

Odometer works by couting the revolutions on the wheel and then multiplying them by 2*pi*R where R is the radius of the wheel.

so unless you changes the tires or have a (very) heavily worn tyre, the odometer is going to be correct by design - you are using your tyre as a tape measure.


speedometer on the other hand works by using the motion of the wheel to generate a current and then sensing the current - higher the speed the larger the current. However the conversion has a lot of variable (strength of the magnet, resistance of the coil, non-linearity, spring in the speedo ...) and hence speedo is an estimate of the speed not the exact speed.

One good thing - speedo error is "stationary" - it wouldn't change much with time. So You can calibrate it - Drive a known length stretch at constant speed (according to speedo) use the stopwatch function on your mobile phone (every mobile phone has it nowadays) and then it is just a matter of D/T.

If you do it at several speeds (10kmph, 20, 30, 40 ...) you'll see that the error may not be same at all speeds.

[vina]

Originally Posted by vina

...


speedometer on the other hand works by using the motion of the wheel to generate a current and then sensing the current - higher the speed the larger the current. However the conversion has a lot of variable (strength of the magnet, resistance of the coil, non-linearity, spring in the speedo ...) and hence speedo is an estimate of the speed not the exact speed.
....

Mustang.101 - I was reading up and found that the new kind of speedometers use Variable Reluctance sensor - this can actually count the number of wheel rotations and coupled with a good clock can give a pretty accurate indication of speed.

In fact on some thread there was a discussion on something called Ford VSS speedometers that are also used to measure wheel speed for ABS.

Does Figo use this kind of speedometer (at least in the ABS version)? If so then the 10% error people are reporting is definitely wrong.

[Mustang.101]

Originally Posted by vina

Mustang.101 - I was reading up and found that the new kind of speedometers use Variable Reluctance sensor - this can actually count the number of wheel rotations and coupled with a good clock can give a pretty accurate indication of speed.

In fact on some thread there was a discussion on something called Ford VSS speedometers that are also used to measure wheel speed for ABS.

Does Figo use this kind of speedometer (at least in the ABS version)? If so then the 10% error people are reporting is definitely wrong.

I do not know the technical aspects of it but ford cars (at least the fiesta, fusion and the figo) do use the VSS sensors for their ABS variants. But i do not have any technical details regarding it.

[aravind.anand]

When cars have odometers that over read, the calculated mileage would obviously higher, thus higher fuel efficiency figures in the minds of users, thus better feedback and word of mouth about the car's fuel efficiency, better sales! Is this a marketing gimmick?

[vina]

Originally Posted by Mustang.101

I do not know the technical aspects of it but ford cars (at least the fiesta, fusion and the figo) do use the VSS sensors for their ABS variants. But i do not have any technical details regarding it.

Thanks Mustang for confirming. Can you check this for non-ABS varients too?

I'll provide the tech brief, detail is here:

VSS is vehicle speed sensor - basically there is a toothed wheel (like a sproket) and a magnetic pickup (like in a tape recorder) that counts the teeth as they go by. It can count rpm at wheel, and then distance is a simple calculation.

New and way-past-discard-now tires will effect the reading by about 1%-2% (up to 5% if somebody is stupid enough to run totally bald tires), I don't think that's even noticeable in FE.

By the way odo can be tested easily with GPS, drive more than 10km and compare odo and GPS - both should match more or less.

[jayded]

Originally Posted by vina

Thanks Mustang for confirming. Can you check this for non-ABS varients too?

I'll provide the tech brief, detail is here:

VSS is vehicle speed sensor - basically there is a toothed wheel (like a sproket) and a magnetic pickup (like in a tape recorder) that counts the teeth as they go by. It can count rpm at wheel, and then distance is a simple calculation.

New and way-past-discard-now tires will effect the reading by about 1%-2% (up to 5% if somebody is stupid enough to run totally bald tires), I don't think that's even noticeable in FE.

By the way odo can be tested easily with GPS, drive more than 10km and compare odo and GPS - both should match more or less.

Hi Vina, if you are asking if a non-ABS Figo has a VSS, I believe it does.
I apparently just got the VSS in my ZXi replaced last month. So it should be present in all variants of the car unless the A.S.S. folks lied to me.

[vina]

Originally Posted by jayded

Hi Vina, if you are asking if a non-ABS Figo has a VSS, I believe it does.
I apparently just got the VSS in my ZXi replaced last month. So it should be present in all variants of the car unless the A.S.S. folks lied to me.

I don't think they would have lied, this is what I was expecting too.

With a VSS on the wheels it is very unlikely that the actual speed will be very different from what the speedo is showing. In fact odo will use the same input.

[wildsdi5530]

A GPS uses instantaneous variation to calculate speed. Aren't the V-box used by vehicle testers GPS based?

[Mustang.101]

Ok guys i did check. All variants of the Figo do use the VSS.

[vina]

Originally Posted by wildsdi5530

A GPS uses instantaneous variation to calculate speed. Aren't the V-box used by vehicle testers GPS based?

VBOX devices are GPS based, but I don't think our friends @wildsdi5530 used VBOX, I think he used a more mundane navigator.

VBOX is fast and very accurate, others may not be that accurate.


Anyway, considering both Fiesta anf Figo use the 175/65/R14 as their default tyre size, using the same VSS (as they most likely do) should result in similar inaccuracy between GPS/speedometer.

However wildsdi5530's experience is different. Which is a little surprising.

[raju2512]

I can't comment about the speedo , but my Figo's odo is correct. I know the distance of few places from my house and the distance noted is the same in my old M800, Wagon R, Ritz and Figo. Recently I drove to Chidambaram with my cosuin in Alto and the distances noted by us were almost the same (with 2-3 km diff over 450 kms)

[vina]

Originally Posted by raju2512

I can't comment about the speedo , but my Figo's odo is correct. I know the distance of few places from my house and the distance noted is the same in my old M800, Wagon R, Ritz and Figo. Recently I drove to Chidambaram with my cosuin in Alto and the distances noted by us were almost the same (with 2-3 km diff over 450 kms)

+1 thanks. If the odo is correct then the speedo can not be very inaccurate (only clock inaccuracy remaincs, and crystal clocks are very very accurate.) because both use the same VSS data for distance measurement.

[wildsdi5530]

@vina, I've checked with the GPS software of my Nokia 5800. I agree that it may not be accurate. But comparing 4 cars, the Honda city ZX, Ford Fiesta, Fusion and Figo, all stock, I've noticed that the city has ~5% error and the figo has ~10% error. And the fusion and fiesta are spot on. Of course you might argue that the figo was spot on and the errors were in the other 3 cars and in my GPS, to which I would have no answer.
I've been using the GPS for over 2 years and more than 80K km.