Speedo Error

[UtahWJR]

Speedo reads and indicates 75mph on the digital part of the guage. Needle appears to match. Using my GPS I get an indicated 70mph. Tach just touching 3000rpm in 5th gear.

Wheels/tires have been upgraded from Factory 17" twists to 18" with factory spec tires (225/40 F and 265/35 R).

Is it common to be off by 5mph? Is there a way to adjust the speedo?

Thanks,

Bill

1999 Boxster

Manual

[1schoir]

Speedo reads and indicates 75mph on the digital part of the guage. Needle appears to match. Using my GPS I get an indicated 70mph. Tach just touching 3000rpm in 5th gear.

Wheels/tires have been upgraded from Factory 17" twists to 18" with factory spec tires (225/40 F and 265/35 R).

Is it common to be off by 5mph? Is there a way to adjust the speedo?

Thanks,

Bill

1999 Boxster

Manual

Bill:

This is not uncommon. If you get the reading directly from the OBDII port, it will match your GPS-generated speed much more closely and only the speedometer will be off.

Take a look at this thread for others who have experienced the same thing: http://www.ppbb.com/phorum/read.php?6,1527705,1527938#msg-1527938

Regards, Maurice.

[infocusf8]

This is common as is the fuel gauge not reading full after you've filled the tank. My speedo reads four mph faster than I'm going according to GPS and those traffic control readouts that are put up to warn you of your speed.

[UtahWJR]

I'm guessing there is no easy way to correct the error?

[wvicary]

Site has a wonderful search function...... just saying.

[infocusf8]

I'm guessing there is no easy way to correct the error?

Have the speedo and the fuel gauge re calibrated. I was quoted $400 and two weeks by a local shop. Instrument cluster has to be removed and sent to a facility that does that kind of work.

[Jinster]

I'm guessing there is no easy way to correct the error?

Have the speedo and the fuel gauge re calibrated. I was quoted $400 and two weeks by a local shop. Instrument cluster has to be removed and sent to a facility that does that kind of work.

Or you can simply go with a taller wall profile tyres next time you change tyres. The speedo is calibrated to the diameter of the tyres.

[infocusf8]

I'm guessing there is no easy way to correct the error?

Have the speedo and the fuel gauge re calibrated. I was quoted $400 and two weeks by a local shop. Instrument cluster has to be removed and sent to a facility that does that kind of work.

Or you can simply go with a taller wall profile tyres next time you change tyres. The speedo is calibrated to the diameter of the tyres.

Since my Boxster came with 17" Pirelli P Rossos, which are a low profile tire, my speedo should be calibrated for those so which replacement tire are you saying would recalibrate the speedo so it is accurate?

[Jinster]

I'm guessing there is no easy way to correct the error?

Have the speedo and the fuel gauge re calibrated. I was quoted $400 and two weeks by a local shop. Instrument cluster has to be removed and sent to a facility that does that kind of work.

Or you can simply go with a taller wall profile tyres next time you change tyres. The speedo is calibrated to the diameter of the tyres.

Since my Boxster came with 17" Pirelli P Rossos, which are a low profile tire, my speedo should be calibrated for those so which replacement tire are you saying would recalibrate the speedo so it is accurate?

Speedo signal comes from axle rotations. Each rotation of the axle is assumed to equal to a certain distance travelled. This distance is the circumference of your tyres. The speedo signal can only measure the rotation of the axle and not the actual circumference of the tyres. So if your tyres have a bigger circumference, your car would cover a larger distance even though the speedo still gets the same amount of axle rotation signal. So if your speedo is off by 5%, then you can just increase your tyre circumference by 5% to offset it. This assumes the increased tyre size fits inside the wheel well - which I would have thought is quite reasonable but isn't something I have experience with personally. I don't know which brand and model of tyre would give you the exact increase in diameter you need. But your tyre shop should have no trouble telling you which ones.

Edited November 23, 2010 by Jinster

[UtahWJR]

I have a device that I can plug into the OBDII port so when the weather is a bit better I see what it says.

Thanks.

[infocusf8]

I'm guessing there is no easy way to correct the error?

Have the speedo and the fuel gauge re calibrated. I was quoted $400 and two weeks by a local shop. Instrument cluster has to be removed and sent to a facility that does that kind of work.

Or you can simply go with a taller wall profile tyres next time you change tyres. The speedo is calibrated to the diameter of the tyres.

Since my Boxster came with 17" Pirelli P Rossos, which are a low profile tire, my speedo should be calibrated for those so which replacement tire are you saying would recalibrate the speedo so it is accurate?

Speedo signal comes from axle rotations. Each rotation of the axle is assumed to equal to a certain distance travelled. This distance is the circumference of your tyres. The speedo signal can only measure the rotation of the axle and not the actual circumference of the tyres. So if your tyres have a bigger circumference, your car would cover a larger distance even though the speedo still gets the same amount of axle rotation signal. So if your speedo is off by 5%, then you can just increase your tyre circumference by 5% to offset it. This assumes the increased tyre size fits inside the wheel well - which I would have thought is quite reasonable but isn't something I have experience with personally. I don't know which brand and model of tyre would give you the exact increase in diameter you need. But your tyre shop should have no trouble telling you which ones.

It would seem then that you would end up with a tire that has a taller side wall and I would question how it would perform under conditions such as track, AX and general aggressive driving since Porsche selects tires that are "performance based" and generally low profile. A new set of tires would also cost as much, if not more, than the $400 calibration so all you would save is time and you might lose some performance.

[Jinster]

I'm guessing there is no easy way to correct the error?

Have the speedo and the fuel gauge re calibrated. I was quoted $400 and two weeks by a local shop. Instrument cluster has to be removed and sent to a facility that does that kind of work.

Or you can simply go with a taller wall profile tyres next time you change tyres. The speedo is calibrated to the diameter of the tyres.

Since my Boxster came with 17" Pirelli P Rossos, which are a low profile tire, my speedo should be calibrated for those so which replacement tire are you saying would recalibrate the speedo so it is accurate?

Speedo signal comes from axle rotations. Each rotation of the axle is assumed to equal to a certain distance travelled. This distance is the circumference of your tyres. The speedo signal can only measure the rotation of the axle and not the actual circumference of the tyres. So if your tyres have a bigger circumference, your car would cover a larger distance even though the speedo still gets the same amount of axle rotation signal. So if your speedo is off by 5%, then you can just increase your tyre circumference by 5% to offset it. This assumes the increased tyre size fits inside the wheel well - which I would have thought is quite reasonable but isn't something I have experience with personally. I don't know which brand and model of tyre would give you the exact increase in diameter you need. But your tyre shop should have no trouble telling you which ones.

It would seem then that you would end up with a tire that has a taller side wall and I would question how it would perform under conditions such as track, AX and general aggressive driving since Porsche selects tires that are "performance based" and generally low profile. A new set of tires would also cost as much, if not more, than the $400 calibration so all you would save is time and you might lose some performance.

Actually, you will end up with a new set of tires with a slightly taller wall profile or a $400 saving.

[infocusf8]

I'm guessing there is no easy way to correct the error?

Have the speedo and the fuel gauge re calibrated. I was quoted $400 and two weeks by a local shop. Instrument cluster has to be removed and sent to a facility that does that kind of work.

Or you can simply go with a taller wall profile tyres next time you change tyres. The speedo is calibrated to the diameter of the tyres.

Since my Boxster came with 17" Pirelli P Rossos, which are a low profile tire, my speedo should be calibrated for those so which replacement tire are you saying would recalibrate the speedo so it is accurate?

Speedo signal comes from axle rotations. Each rotation of the axle is assumed to equal to a certain distance travelled. This distance is the circumference of your tyres. The speedo signal can only measure the rotation of the axle and not the actual circumference of the tyres. So if your tyres have a bigger circumference, your car would cover a larger distance even though the speedo still gets the same amount of axle rotation signal. So if your speedo is off by 5%, then you can just increase your tyre circumference by 5% to offset it. This assumes the increased tyre size fits inside the wheel well - which I would have thought is quite reasonable but isn't something I have experience with personally. I don't know which brand and model of tyre would give you the exact increase in diameter you need. But your tyre shop should have no trouble telling you which ones.

It would seem then that you would end up with a tire that has a taller side wall and I would question how it would perform under conditions such as track, AX and general aggressive driving since Porsche selects tires that are "performance based" and generally low profile. A new set of tires would also cost as much, if not more, than the $400 calibration so all you would save is time and you might lose some performance.

Actually, you will end up with a new set of tires with a slightly taller wall profile or a $400 saving.

If this is correct then why doesn't Porsche and the independent shops recommend it as a cure for speedo calibration? A taller wall profile is not necessarily a good thing on a high performance vehicle used in racing conditions. If all you want to do is drive on the street then possibly.

[Jinster]

I'm guessing there is no easy way to correct the error?

Have the speedo and the fuel gauge re calibrated. I was quoted $400 and two weeks by a local shop. Instrument cluster has to be removed and sent to a facility that does that kind of work.

Or you can simply go with a taller wall profile tyres next time you change tyres. The speedo is calibrated to the diameter of the tyres.

Since my Boxster came with 17" Pirelli P Rossos, which are a low profile tire, my speedo should be calibrated for those so which replacement tire are you saying would recalibrate the speedo so it is accurate?

Speedo signal comes from axle rotations. Each rotation of the axle is assumed to equal to a certain distance travelled. This distance is the circumference of your tyres. The speedo signal can only measure the rotation of the axle and not the actual circumference of the tyres. So if your tyres have a bigger circumference, your car would cover a larger distance even though the speedo still gets the same amount of axle rotation signal. So if your speedo is off by 5%, then you can just increase your tyre circumference by 5% to offset it. This assumes the increased tyre size fits inside the wheel well - which I would have thought is quite reasonable but isn't something I have experience with personally. I don't know which brand and model of tyre would give you the exact increase in diameter you need. But your tyre shop should have no trouble telling you which ones.

It would seem then that you would end up with a tire that has a taller side wall and I would question how it would perform under conditions such as track, AX and general aggressive driving since Porsche selects tires that are "performance based" and generally low profile. A new set of tires would also cost as much, if not more, than the $400 calibration so all you would save is time and you might lose some performance.

Actually, you will end up with a new set of tires with a slightly taller wall profile or a $400 saving.

If this is correct then why doesn't Porsche and the independent shops recommend it as a cure for speedo calibration? A taller wall profile is not necessarily a good thing on a high performance vehicle used in racing conditions. If all you want to do is drive on the street then possibly.

This is correct as far as correcting the speedo error goes in theory.

Why Porsche doesn't recommend this is not for me to know. Don't put too much faith in Porsche. They are the ones that made the mistake in the first place. Behind the corporate facade and a brand name that might instill confidence in the customer, there are a still a bunch of human engineers of various levels and experiences working on things. They make mistakes and omissions like anyone else. However, one possible suggestion, like Maurice says, is that it is only the speedo that is off, not the OBD2 data as used by the ECU. This almost suggests deliberate foul play by Porsche. Be it as it may. Another possibility, is that the fatter tires may cause problems with a lowered car. A third possibility, is that Porsche simply doesn't want to admit to the problem.

I would place even less confidence in independent shops. I would've thought that the speedo correction shop would propose the speedo correction method rather than the fatter tire method for obvious reasons, amongst others.

Yes, it is possible that tires with a taller wall profile might not be better for racing.

Edited December 12, 2010 by Jinster

[infocusf8]

I'm guessing there is no easy way to correct the error?

Have the speedo and the fuel gauge re calibrated. I was quoted $400 and two weeks by a local shop. Instrument cluster has to be removed and sent to a facility that does that kind of work.

Or you can simply go with a taller wall profile tyres next time you change tyres. The speedo is calibrated to the diameter of the tyres.

Since my Boxster came with 17" Pirelli P Rossos, which are a low profile tire, my speedo should be calibrated for those so which replacement tire are you saying would recalibrate the speedo so it is accurate?

Speedo signal comes from axle rotations. Each rotation of the axle is assumed to equal to a certain distance travelled. This distance is the circumference of your tyres. The speedo signal can only measure the rotation of the axle and not the actual circumference of the tyres. So if your tyres have a bigger circumference, your car would cover a larger distance even though the speedo still gets the same amount of axle rotation signal. So if your speedo is off by 5%, then you can just increase your tyre circumference by 5% to offset it. This assumes the increased tyre size fits inside the wheel well - which I would have thought is quite reasonable but isn't something I have experience with personally. I don't know which brand and model of tyre would give you the exact increase in diameter you need. But your tyre shop should have no trouble telling you which ones.

It would seem then that you would end up with a tire that has a taller side wall and I would question how it would perform under conditions such as track, AX and general aggressive driving since Porsche selects tires that are "performance based" and generally low profile. A new set of tires would also cost as much, if not more, than the $400 calibration so all you would save is time and you might lose some performance.

Actually, you will end up with a new set of tires with a slightly taller wall profile or a $400 saving.

If this is correct then why doesn't Porsche and the independent shops recommend it as a cure for speedo calibration? A taller wall profile is not necessarily a good thing on a high performance vehicle used in racing conditions. If all you want to do is drive on the street then possibly.

This is correct as far as correcting the speedo error goes in theory.

Why Porsche doesn't recommend this is not for me to know. Don't put too much faith in Porsche. They are the ones that made the mistake in the first place. Behind the corporate facade and a brand name that might instill confidence in the customer, there are a still a bunch of human engineers of various levels and experiences working on things. They make mistakes and omissions like anyone else. However, one possible suggestion, like Maurice says, is that it is only the speedo that is off, not the OBD2 data as used by the ECU. This almost suggests deliberate foul play by Porsche. Be it as it may. Another possibility, is that the fatter tires may cause problems with a lowered car. A third possibility, is that Porsche simply doesn't want to admit to the problem.

I would place even less confidence in independent shops. I would've thought that the speedo correction shop would propose the speedo correction method rather than the fatter tire method for obvious reasons, amongst others.

Yes, it is possible that tires with a taller wall profile might not be better for racing.

Theory and practicality are two different animals. I would want to discuss the results with someone who has done it and can show it works before I altered the existing dynamics of my Boxster. For all the criticism Porsche takes for their design criteria the engineers have really designed a very good car.

[tholyoak]

If the built in error in the displayed speed bothers you, there are several simple electronic devices you can install for <$100 to calibrate the signal received by the cluster so that the actual speed displayed in the speedo matches the actual speed of the vehicle as displayed in the DME.

-Todd

[Jinster]

Theory and practicality are two different animals. I would want to discuss the results with someone who has done it and can show it works before I altered the existing dynamics of my Boxster. For all the criticism Porsche takes for their design criteria the engineers have really designed a very good car.

Do what you like, mate. Why shouldn't you? Porsche made a good car to be sure, but they didn't get the speedo right.

Todd is correct. Such devices have been available for a while. Speedo signals are generated by turning a magnetic gear mounted on the wheel axle, thus generating a square wave signal with each passing gear, corresponding to the wheel turn - similar to using an AC motor in reverse. These correctors work by changing the square wave input frequency of the speed signal by a certain percentage and spits out the new square wave at the other end.

I don't know whether it'd work on the Boxster though. Has anyone tried it? There are four speed sensors, one at each wheel. The non-PSM/TC ABS system is supposed to compare the signals and throw an error code if the speeds don't match within a certain tolerance. Also, if you have PSM/TC, the wheel speed differential is what triggers biased braking. To do this correction "properly", you will need four such correctors. It may be possible to just use one corrector on the final output of the ABS system, but I don't know if the signal would have been converted to a voltage/current by that stage - certainly doable if it's still a frequency signal. But given the odometer is also in the instrument cluster, this might reduce your true odometer reading if it is true that the odometer displays the correct mileage and it's only the speedo that's off - this may or may not be a problem for some.

Another possible way to correct this that some speedometers have built-in calibration resisters near the needle mechanism. Anyone seen one of these when they opened their instrument cluster?

I will take mine off at some stage soon when I mount the OBD stalk. I will try to spot such a resister, if I remember.

Personally, I always found the visual accommodation required to look at the speedo distracting, especially as I drive 700km a week for work in my Boxster. I have installed HUDs in all my cars for comfort and I calibrate them to radar detector speeds, which seems to be 10% lower than the speedo indicates at all legal driving speeds.

Edited December 14, 2010 by Jinster

[infocusf8]

Theory and practicality are two different animals. I would want to discuss the results with someone who has done it and can show it works before I altered the existing dynamics of my Boxster. For all the criticism Porsche takes for their design criteria the engineers have really designed a very good car.

Do what you like, mate. Why shouldn't you? Porsche made a good car to be sure, but they didn't get the speedo right.

Todd is correct. Such devices have been available for a while. Speedo signals are generated by turning a magnetic gear mounted on the wheel axle, thus generating a square wave signal with each passing gear, corresponding to the wheel turn - similar to using an AC motor in reverse. These correctors work by changing the square wave input frequency of the speed signal by a certain percentage and spits out the new square wave at the other end.

I don't know whether it'd work on the Boxster though. Has anyone tried it? There are four speed sensors, one at each wheel. The non-PSM/TC ABS system is supposed to compare the signals and throw an error code if the speeds don't match within a certain tolerance. Also, if you have PSM/TC, the wheel speed differential is what triggers biased braking. To do this correction "properly", you will need four such correctors. It may be possible to just use one corrector on the final output of the ABS system, but I don't know if the signal would have been converted to a voltage/current by that stage - certainly doable if it's still a frequency signal. But given the odometer is also in the instrument cluster, this might reduce your true odometer reading if it is true that the odometer displays the correct mileage and it's only the speedo that's off - this may or may not be a problem for some.

Another possible way to correct this that some speedometers have built-in calibration resisters near the needle mechanism. Anyone seen one of these when they opened their instrument cluster?

I will take mine off at some stage soon when I mount the OBD stalk. I will try to spot such a resister, if I remember.

Personally, I always found the visual accommodation required to look at the speedo distracting, especially as I drive 700km a week for work in my Boxster. I have installed HUDs in all my cars for comfort and I calibrate them to radar detector speeds, which seems to be 10% lower than the speedo indicates at all legal driving speeds.

Well they also didn't get the gas gauge right either and after spending almost $900 to correct mine, and it still isn't correct, I am hesitant to spend any more money on gauges until someone can prove what they've done is effective. I'm amazed that we used to get correct fuel gauge readings with a toilet float and armature and the "improved" electronics can't be accurate. If my Boxster is actually going 4-5mph slower than what is indicated on my readout perhaps it could be a good thing in instances where I'm being tagged by radar.