i was led to believe that Porsche speedometers are pretty accurate, whereas say, Italian ones are overly optimistic.

this theory was proven somewhat, as I GPS'd my friend's Lambo at around 186mph, when the speedometer said 200mph.

and then some time ago i did a GPS'd 200+ mph run in the RT12, and the car's speedometer was only about 1-2 mph off.

however yesterday morning, the GT3's speedometer said 188mph, and my GPS indicated only 180mph. at lower speeds, say 100mph, the variance was much less, understandably, around 3mph.

both the Ruf and the GT3 are 997's, and both running OE tires.

not that this bothers me or is a problem, am just curious why the very different behaviors? or is this just down to tolerances in calibration across different cars?

 

Nearly all production cars speedometers are optimistic, and they are off by a percentage, so as the speed increases, the difference in MPH increases. At legal speeds, most are off by 1-3 mph. At the speeds you are talking about, I am not surprised at all with what you are seeing.

Nobody would consider RUF a mainstream production car, so their accuracy is not surprising.

Any vids of the runs? I'm sure many here would love to see them!

 

some of the vids have been on youtube for some time actually. i've got my flamesuit on for being unsafe and all, but do note the runs were done with spotters up ahead, and the Ruf run was done with consent of the highway patrol (long story, but living in a banana republic has some benefits )

Lambo run

Ruf

997.1 GT3 yesterday

 

My GT2's speedometer said 332, but the Driftbox GPS said 319..

 

looks like the 12-13kmh difference at high speed is about consistent then!

 

I think my last three cars were about 6-7mph higher at a true 80mph.

 

it has been said that the lambo speedometers are 10% off above 40mph....which is the legal limit

 

OK, just to clear things up. The error is cumulative. So of course the faster you go the bigger the error. As long as the speed is picked up from the wheels and not from GPS you have to make sure your tires are properly inflated and you run stock wheels AND tires as you may have a different circumference than originally installed. The deviation from one car has absolutely nothing to do with another. Gauges in an Italian or German car could have been made anywhere, you would have to check the manufacturer.

 

Actually, not all speedos are off by very much. Most MB and Audi speedos are within 1-2% of true speed (at least the 3 MBs and 2 Audis I measured were). At 200mph, a 1% error would be 2mph, not 14. Speedometers, being recording devices, should at least be precise if not accurate (ie. if there's an inaccuracy, it should be predictable).

Of the four Porsches I've owned, two had speedos that were off by exactly 10%, verified by repeat passes by a calibrated radar gun. The other two were off by about 4.5%, again verified by a radar gun. I know this because it was a feature of the cars that miffed me from day one. With a motorcycle racing background (albeit limited and amateur), I have a pretty good seat-of-the-pants feeling for how fast I'm going, and I was frankly embarrased to have cars that cost as much as Porsches with instruments less accurate than a $35K Buick. I got the Porsche regional rep involved and even wrote letters to PCNA; usually I got no response, but the one response I did get is that in order to comply with European laws, the speedos can't underreport speed, so they err on the side of overreporting. Again, MB seems to have figured out how to put accurate instruments in cars, but in spite of the things Porsche does right, putting in accurate speedos isn't one of them.

Ultimately, I don't think the speedo issue is grounds for not driving the cars if you like them, but it's probably not a bad idea to have your speed accurately measured so that you know how fast you're really going. It matters to the "Five-O".

 

Your RT12 sounds nice!