Solution for speedo error on 996 turbos.
Discussion
This has been my only annoyance with our car. When we go to Vmax the speedo always over-reads hugely, perhaps by 15mph sometimes.
Anywho i have been driving in Europe for the last couple of days and have had the digital speedo in kmh. Whilst driving through france at a fairly decent clip i noticed that the analogue speedo(which is still in mph) was showing a much higher speed than the kmh digital reading.
So last night i conducted a little test. Speed up to 186mph on the digital speedo and then switch to kmh(easily done i had set everything up beforehand) and the kmh display came up, reading 279kmh.(172mph)
What do you guys think? Have Porsche FUBAR'd with their mph equations or something?
all the best
adam
Anywho i have been driving in Europe for the last couple of days and have had the digital speedo in kmh. Whilst driving through france at a fairly decent clip i noticed that the analogue speedo(which is still in mph) was showing a much higher speed than the kmh digital reading.
So last night i conducted a little test. Speed up to 186mph on the digital speedo and then switch to kmh(easily done i had set everything up beforehand) and the kmh display came up, reading 279kmh.(172mph)
What do you guys think? Have Porsche FUBAR'd with their mph equations or something?
all the best
adam
Adam,
Does your car have factory-spec tyres on it? Don't Ruf use a very low aspect-ratio tyre? That would increase the rate of over-reading.
On the GT3-II, indicated 203 is actual 190.
On the CGT, indicated 217 is actual 206.
Both were on standard Pilot Sports that were about 1/3 worn.
Does your car have factory-spec tyres on it? Don't Ruf use a very low aspect-ratio tyre? That would increase the rate of over-reading.
On the GT3-II, indicated 203 is actual 190.
On the CGT, indicated 217 is actual 206.
Both were on standard Pilot Sports that were about 1/3 worn.
The error varies from car to car.
My car at the last VMax was indicated at 200/201mph through the timing beam and was doing an actual 196mph, so it's very close. Tyres are stock sizes, but Pirelli Corsas.
I would have thought that the ecu could be adjusted using the Porsche diagnostic/programming tool.
The error between km and mph seems odd though.
Guy
My car at the last VMax was indicated at 200/201mph through the timing beam and was doing an actual 196mph, so it's very close. Tyres are stock sizes, but Pirelli Corsas.
I would have thought that the ecu could be adjusted using the Porsche diagnostic/programming tool.
The error between km and mph seems odd though.
Guy
GuyR said:Guy,
The error varies from car to car.
My car at the last VMax was indicated at 200/201mph through the timing beam and was doing an actual 196mph, so it's very close. Tyres are stock sizes, but Pirelli Corsas.
I think you'll find that, for a given set of nominal dimensions, from one maker to another there is considerable variation in the actual dimensions - both rolling diameter and width.
flemke said:
Guy,
I think you'll find that, for a given set of nominal dimensions, from one maker to another there is considerable variation in the actual dimensions - both rolling diameter and width.
Agreed. I recall checking the rolling circumference for the Corsas though and they were very similar to the Michelin Pilots that were fitted, if not identical. I did VMax with both sets of tires and wasn't getting any noticeable difference in speedo readings, well at least not of the order of Adams, whose car has shown 15-20mph over-speed.
>> Edited by GuyR on Monday 20th June 13:58
adamt said:Adam,
We have crappy Yokohama AVS Sports, which are 315/25 19 at the rear.
What was the CGT like at 206mph? Was it bouncing off the limiter?
adam
If you run the numbers I think you'll find that, in theory, the Yokohamas have about 2% greater diameter than what I think is standard on the 996TT (285/30/18?).
The CGT was pretty much at its true max speed. As I said in the article, there may have been another 1 mph in it, but I had to back off for traffic, which had started to build (plus I had to allow time for Roger Green to do some runs). Maybe the true max for the car is 207, but maybe not. Then you get into following or head winds, uphill or down, etc. Who knows?
The rev limit seems to be at about 8200. I'd say that in sixth we were at around 8000-8100. I was not focusing on the tach at the time.
Cheers.
Excellent stuff, i sadly missed the EVO issue, and havent read the article, but thank you for your account it is really appreciated.
I love chasing the Vmax of these cars, because it is so exciting, and also there is always that nagging feeling that it could have gone a bit faster!
all the best
adam
I love chasing the Vmax of these cars, because it is so exciting, and also there is always that nagging feeling that it could have gone a bit faster!
all the best
adam
Stock Turbo is 295/30/18 which on PS2 is 832 revs/mile, whereas 315/25/19 is 824 revs/mile (source Tirerack.com). This is a 1% error, so would only account for 2pm at 200mph, not the 15-20mph difference that Adam is seeing.
The speed sensor is presumably from the ABS. The question is whether the front or rear wheels are used, or an average. This would dictate which tire diameter changes (front or rear or both) affect the speedo reading.
PS The Evo article was great. I have every issue of Evo published, it's the only mag I keep of the dozens I buy each month.
>> Edited by GuyR on Monday 20th June 15:11
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