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    Suspect Speedo

    Worrying, getting overtaken all the time by vans & ????? while trying to stay in speed limit +10 k, over first 3 weeks thought my car felt really slow compared to registered speed. Today got flashed by one of those radar "Check your Speed" before roadworks on motorway signs & was only doing 91 kph while speedo registered 119kph, other car overtook me registered 133kph. Amazed, turned around & came past again, car well in front registered 118kph & me 89kph but my speedo was showing 116kph.

    This explains why got almost 41mpg on 200k trip & my mileage recording seems high, but why?? tomorrow, before confronting dealer will check against another car on dual carrigeway @ 80 kph 100 kph &.

    Can this have someting to do with 18" S wheels? Speedo not recalibrated? anybody else come across this? Thought I was going daft doing 120kph behind trucks that I knew could not all be breaking the speed limit (100kph) also even in traffic at crawl registering 14-18kph. Changing digital readout to MPH just the same. Help.

    Re: Suspect Speedo

    Pols, something's not right. If you got 41mpg, it would be because your odometer is reading more kilometers than reality, not less than reality. Think about it. BTW, my 2004 Boxster speedo reads 3 mph higher than the "check your speed" machines say.

    Re: Suspect Speedo

    That day was 40.9 amazing - today was 35.8, but if I am only driving at 80 - 90 kph maybe?, but I have also pushed my car as in nears 2400ks, I told my dealer after first week car felt slow compared to speedo read out but even then I thought that was just me getting used to different car. Mind you, looking at last months petrol receipts not so sure about that mpg anymore either, something very very wrong here. S**t. Because of Galway race week here, dealer closed till next Monday.

    Re: Suspect Speedo

    Hi Pols,

    I don't know what is wrong, but your logic about the gas mileage and the inaccurate speed readout is internally consistent. In essence your car is overestimating the distance it is traveling. So your odometer probably has more kms than you have actually driven, your speedo will show a higher speed than you are actually traveling, and your fuel mileage will be calculate to be higher than it truly is. I am not sure it is a wheel issue though, since the circumference of all the the 18 and 19 inch whell/tire combos are within few percent of one another. I don't know about the standard Boxster 17 inch circumference though. Anyway, this sounds like a problem that would be easy to demonstrate to your dealer. Just be sure to have them make some sort of adjustment to your warranty if in fact the odometer is showing more kms than you have actually traveled.

    mcdelaug

    Re: Suspect Speedo

    Whoops. Mcdelaug is right Pols. I read your post again and it made sense this time. Tire and wheel sizes dont vary as much as your speedometer is off. There's a simple 3 step fix, however. Remove old speedo. "Adjust" old speedo with a 2kg hammer. Install new speedo.

    Re: Suspect Speedo

    Yep, sounds good to me, have just the right hammer too. Apart from the odometer it is quite funny really, here's me in my Porsche @120ish ks not pushing it (speed camera watch) while in fact only tootling along @ about 90ks & being overtaken by a "Lada" Duh!

    Re: Suspect Speedo

    I had a similar thing happen on my old Golf - after changing the instrument section, they had forgotten to programme it for wheel diameter correctly. Same symptoms as you describe. But aren't even the 17"s the same diameter overall as the 18"s and 19"s?... Haven't got my specs to hand. Get 'em to do a diagnostics. Let us know, as it's definitely strange!

    You haven't got flat tyres have you?

    Re: Suspect Speedo

    Damm, that is why the car is so low & noisy. All I need is air.

    Re: Suspect Speedo

    I checked mine against my GPS (not PCM) system and the speedo overreads by about 2mph - but most UK cars are set up that way as they cannot underread and be legal.

    As said earlier it probably isn't wrongly programmed wheel diameters as there isn't that much difference between the outside diameter of 17, 18 or 19 inch wheels.

    A lot of modern cars use the ABS wheel sensor as a speed sensor and these are usually non-contact sensors - a popular type uses a hall (magnetic) sensor placed quite close (a few mm) to a metal disc (can be part of the wheel hub) with notches in it and every time a notch goes past the sensor generates an electrical pulse and the speedometer basically counts the pulses. If the disc is slightly out of round or the sensor spacing is wrong or if there is rubbish in the notches or on the disc surface then this can generate extra pulses which would cause an overread.

    Having said this I would expect an overread to possibly have other side effects, particularly on PSM as four of the inputs to that should be the individual wheel speeds - unless it only looks for changes in wheel speeds.

    Whatever it is it definitely needs to visit the dealer!

    Re: Suspect Speedo

    I agree - the speed readout on my Origin B2 (at constant speed to try and avoid the slight delay on GPS readout and the car speedometer) is about 2 mph at 80 (sorry 70 ) mph.

    Re: Suspect Speedo

    Sorry, can't help it but the thought of a typical driver conversation just makes me giggle. "So, what's your average gas mileage on your Porsche?" - " Oh around 40, when I drive briskly." - "!!!"

    Lucky though this happens early on and can be fixed under warranty. Keep us posted.

    Re: Suspect Speedo

    I spoke to OPC this morning & seems is a known fault over here on 987 & 997 cars where we have KPH, I bought my car from my local specialist dealer but will always be serviced or warranty faults by OPC in Dublin.Problem something to do with calibration ??? BS I think as cars have MPH speedo & digital KPH (choice) UK Spec. Anyway my car will be transported to OPC Dublin next week & sorted. Pain In A*s, no doubt will get new crappy SAAB diesel as replacement while OPC get around to fixing issue & the sun shines.

    Re: Suspect Speedo

    Quote:
    pols said:
    I spoke to OPC this morning & seems is a known fault over here on 987 & 997 cars where we have KPH, I bought my car from my local specialist dealer but will always be serviced or warranty faults by OPC in Dublin.Problem something to do with calibration ??? BS I think as cars have MPH speedo & digital KPH (choice) UK Spec. Anyway my car will be transported to OPC Dublin next week & sorted. Pain In A*s, no doubt will get new crappy SAAB diesel as replacement while OPC get around to fixing issue & the sun shines.



    It is BS - you can switch the digital readout yourself from kph to mph using the onboard computer menu.

     
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