CraigyB

Also, I notice the difference when slowing for the gatso in my Audi, that I tend to slow to a similar speed to the rest of the traffic, but with the porker, end up going slower than most other cars if I watch the speedo !

911addict

Craig,
Could be, like me, in the Porsche you get paranoid and feel you'll be the one to get nicked, so you make sure you within the speed limit, whereas the other drivers know that most cameras have a 10% allowance and go that 10% faster.

TR6

Because the satellites are so far away in distance relative to the gain or loss in altitude of your car, the differential angle is so minute as to be irrelevant to the speed measurement indicated by the GPS. Sophomore geometry. More or less, kind of the same reason the full moon seems to travel alongside you as you drive.

psychoideas

Hi All,

Although most aircraft now have GPS I thought that they all must have an Air Speed Indicator which measures the speed of the air passing the plane?
Anyway, not my forte`.

As far as speed in the car goes I use the time over distance method.

Distance (in metres) divided by Time (in seconds) multiplied by 3.6 = Kilometres per Hour

I’m sure the conversion to imperial is pretty straight forward.


In Victoria Australia we have Kilometre Posts on the side of the Freeway every Kilometre. They measure the distance from the General Post Office (GPO) in Melbourne.
So, as you pass the 220Kilometre Post it means you are 220Km from Melbourne.
(It’s to assist Emergency Services to find reported accident/incident scenes.)

I use these to check my speedo. If I sit exactly on 100Kph between the posts it should take me 36 seconds. Any different and I work it out on the calculator in my phone.

Good luck,

Frank

jjbunn

So if the speedo is calibrated for 16" wheels at the factory, and the car actually has 18" wheels fitted, then that would be a 2/16 or 12.5% error ...

And, with the bigger wheels, the speedo would tend to under-estimate the real speed, since one revolution of the big wheel covers more ground than one revolution of the 16" wheel.

Or do I have it all wrong?

JasonAndreas

Does anyone with a MY91+ also have GPS? For MY89 and MY90 Porsche fed the speedo with output from the transmission. The speedo output from the ABS controller (MY91+) seems to match the radar warning at most of the exits on the Massachusetts turnpike upto at least the speed limit (65MPH) and probably higher...

Christer

Hell yeah you have it wrong The use of a larger wheel but a lower profile tyre ensures that the rolling diameter stays *reasonably*constant therefore it makes little difference. For example, in my experience the GPS showed my car with 17" wheels fitted was going around 5mph slower than what the speedo showed. When I switched to 18" wheels, the difference is slightly more but not as much as 6mph slower...

jimq

mine is and i do. As above 60 on car 58 GPS

jjbunn

Hmmm so that is weird ... I'd expect the speedo to *under* estimate the real speed, with the larger wheels.

Johnny G Pipe

Yep. The datalogger from www.race-technology.com does that stuff. There was one being passed around the uk rennlist 964 gang a couple of years ago which resultred in lots of 0-60's in mid-low 5's if I recall..

TR6

Would that data logger from the UK also work in the US, I wonder? Are GPS systems globally compatible?

Has anyone ever used one of these? http://www.gtechpro.com/

The 0 - 60 acceleration discussion is interesting to me. I have seen road test results for the RS America with 0 - 60 as low as 4.6 seconds (see http://www.rsamerica.net/articles/ro...Car&Driver.htm ). Another road test put it at 4.93 seconds (see http://www.rsamerica.net/articles/roadtest/SCI.htm ). But many others quoted 5.4 seconds (which I read somewhere was not actually from independent testing, but just quoting the official factory information which was probably conservative). I would love to get some real data from real field testing to show what our 964's acheive in 0-60 times. Any of you guys have any real data on this?

DaveK

I believe that the GPS satellites actually belong to you guys - the rest of us just borrow them.

frankvan

one could assume that something known as the "Global Positioning System" would be globally compatible

In a nutshell: originally set up by US military in the 70's with the establishment of 24 (I think) satetllites that transmit a "time code". The GPS receiver takes the differences between the time codes from different satellites and triangulates your position on the earth.
Made available to public in 80's although the "P" codes - no, that doesn't stand for our favourite german cars - remain classified. The P-codes allow accuracy within 1 meter theoretically. THe civilian available signal is still very precise but accuracy can be around the 100 meter mark - but with the P code one never knows! (the code causes the accuracy to "randomly" wander so you code be exactly where it says you are or up to 100 meters away). This accuracy problem can be combatted with a system called d-GPS (d=differential) which uses a land based stationary reference tranceiver. This can be v/ important for things like side scan sonar surveys of the ocean floor....
I do believe in recent years with the advent of GPS navigation in cars and commercial aircraft that the US has reduced this built in error but one never knows by how much
(just my $0.02 and recollection from time served as a ship's navigator)

DaveK

Ah, but if you believed that then you'd think the World Series included the countries of the world.......

SimonExtreme

Accuracy of GPS is, these days, a lot better than within 100 metres. There have been a couple of changes made to what is available to the public and now it depends on how much you are prepared to pay for the equipment. There are various ways of enhancing GPS calculations and it is possible to get systems that measure to within 20-30 cm!

Depending on use, there are all sorts of enhancement techniques. Mentioned above, GPS for aircraft is enhanced by Local Area Augmentation System (LAAS) which works up to 30km from source and is used at airports. So, if you are near an airport, your GPS might be more accurate if your system can use LAAS!

Now, before you all think I am a geek, I looked iot up on the web because there are occasions when I trust my life to GPS accuracy (ski mountaineering) and 100m accuracy means the difference between cliffs or crevaces!