BetterMakeWay

But that would mean he would have lost speed instantly...at least those precious last km/h that are so hard to achive. I mean it's hard to bealive that he lift off the throttle and in that instant took the picture loosing no speed. Or could be...DANG i wish i had an ecu remap now.

Any info about how to go to that secret menu?

dlevi67

No way he's getting 15 l/100 km at that speed. Alex is right. Also take into account that - digital or not - there is a little smoothing, so instrument response is not instantaneous.

Secret menu: search for "hidden" "menu" on all forums here.

big_ipaq

Sorry, I don't get it. GPS shows 268 km/h. Instrument shows 258 km/h and speedometer shows 260 km/h in different picture, which I believe are the EXACT same speeds, differently displayed. My experience shows that at 200 km/h in my car with original tires, actual average speed is around 194-195 km/h if not less.

What has GPS indicated speed got to do with speeds in the other pictures? Are they on the same car? At the same speed? At the same moment?

I guess nothing to do. Also, GPS speed is not true speed. Is calculated using position difference between 2 points relative to 1 second intervals which commercial GPS receivers can achieve. You need constant speed to calculate accurately the speed, AND proper software to interpret the data.

Otherwise, I may have 250 km/h with the car stationary, if succesive positioning points will be affected by poor GPS signal induced positioning errors.

dlevi67

GPS and digital instrument both show 268. Look again. The speedo shows what it can - i.e. top of scale. Given that this is not a court of law, I think it's a reasonable assumption that the GPS and the instrument binnacle pictures are of the same car at the same speed in roughly the same moment; your seeming assumption that they are not is equally valid, but not as interesting (what would be the point?)

Yes, the speedo speed and the true speed will be different by ~2%. The digital instrument shows two speeds: the true speed (to the limit of accuracy possible, e.g. tyre diameter variation due to wear), and the speedo indicated speed (plus the set cruise control speed), with roughly a 2% variation in favour of the speedo.

Re: GPS accuracy - that's the definition of (average) speed - space over time. If your speed varies significantly in one second it won't be accurate, but given GPS's accuracy of 1.5/2m, the speed is correct within 5-7 kph even if you are changing satellites (GPS timing is about as accurate as you can get).

Your assertion that you can get a 250 kph GPS speed read with a stationary object is completely absurd - GPS clocks are accurate and synchronized to one part in several million; therefore time differences are negligible, so any "artificial" speed differences are due to location precision differences. 250 kph is nearly 140 m/s; this would imply that GPS cannot locate your car within a radius of 140 m, which it clearly can!

nicke60gre

Or, that picture is taken on a dyno

big_ipaq

Sorry. Two separate pictures are not evidence. Even if GPS would be in the same picture as spedo and instrument speed would not be evidence. At least not for me. One picture still contains EXIF info, one not, just to note first discrepancy.

You say:
This is simply not true. The GPS accuracy is not 1.5/2m, at least not now, not everywhere, not with all receivers.

Typically a normal GPS receiver, has accuracy of 30-40 m IN IDEAL CONDITIONS.

If sky conditions, and obstructions are blocking GPS signal, this can DRAMATICALLY reduce accuracy to 100-300 m easily. It will also depend on azimut and apparent altitude above horizon of the tracked satellites and also on the number of tracked satellites. There are too many factors.

If the signal is augmented, by any form of DGPS, then the typical accuracy is 10-15m. Note I am using typical, there are NO GUARATEES.

In Europe, EGNOS wide area augmentation geostationary satelites, can improve the GPS singal to values close to what you are talking. Assuming EGNOS sattelite is visible and proper signal is received, and you're not to far away from the DGPS point relevant to your position, broadcasted by EGNOS sattelite. However EGNOS are just in test, and IIRC they will be functioning in stage 1 of Galielo in 2008.

So, to believe that GPS value as very close to true groundpseed, I should know, how many visible sattelites is the receiver tracking, if the signal is augmented and what's the correction, and that the value is hold over at least 10-15 seconds. Of course, good software, can filter out positioning errors, based of position points history. If you're going with a car is hard to accelerate from 0 to 268 km/h next second. However these are assumtions that good quality receivers are not making.

And yes, I know very well many things about GPS system, and I've seen many big inaccuracies. I've also tested with EGNOS augmented signal back in 2004.

Please, don't ever mention that accuracy is 1,5-2 m. The correct reply, would be: "in ideal conditions, with external augmentation signal, the probability of <3m accuracy is above 90%".

big_ipaq

Indeed, you are right, I misread it first time, but it does not make me believe.

BetterMakeWay

I bealive it. Why? Just because that 268km/h might not be the real speed but no instrument cluster or gps device can measure the exact speed as the pro gagets and radars used for it. So if we are already considering a stand point the offset of the speedo readings than it's all ok. We don't have to know EXACTLY the real speed of the car at 268km/h on the instrument. It's just enough that the instrument read that speed as well as speeds as 140,190,220km/h and so on.

big_ipaq

I do not agree. Just changing the wheels will affect the speed value. Not to mention, the calibration is always needed. I've seen cars with 20-40 km/h offset, because of some problems.

So a picture, does not do it for me.

BetterMakeWay

Yes but we are starting with the no-guilty presumption, and well, knowing that he didn't change the wheels or he didn't make any modifications, knowingly, that could improve or cheat on the readings then it's all ok.