Speedo Inaccurate

[Beaker]

I assume the speedo is electronic? If so there has to be a way (for someone who knows what they are doing) to rewrite the code so that the speedo displays the same speed as the ECU is seeing?

Anyone? Maybe we could get one of the aftermarket tuners to add some code to the remap?

[Corey_R]

Beaker - that's what Maverick and Logger have been discussing in previous pages, changing settings in the VAGCOM software. But it seems that the only adjustment is some type of multiplier which affects several values at once.

I don't believe that ECU reflashes change the code which handles the speed - but logger or Guy would be better people to comment on that.

[Beaker]

Fair enough I have 0 experience with flashing standard ECUs all my experience is with aftermarket ECUs and custom maps. None of those had to do anything other then run the engine.

[SteveK]

I've tried tweaking the Distance Impulse Number because on the R with 19s I'm doing 94-95kmh when the needle and digital speedo are showing 100kmh. I want to know exactly what I am doing when I look down, not some mathematical acrobatics. The best I could do was set it to Distance Impulse Number 8 and the country to Australia and now I'm only about 3kmh out.

Cheers,
Steve

[Corey_R]

I've also tried tweaking the Distance Impulse Number because on the R with 19s I'm doing 94-95kmh when the needle and digital speedo are showing 100kmh. I want to know exactly what I am doing when I look down, not some mathematical acrobatics. The best I could do was set it to Distance Impulse Number 8 and the country to Australia and now I'm only about 3kmh out.

Keep in mind now that you've done that, that your entire trip meter is now wrong, including fuel consumption and Odometer. The car KNOWS that it's doing 93/94km/h, but tells you that you're doing 100km/h because it's required to do that by law. If you were to do 100km/h and then reset your trip meter, it would then tell you CORRECTLY that you're averaging 93/94km/h. Now that you've changed those figures, it'll give you incorrect information, incorrect fuel consumption, incorrect range til empty, etc etc.

[Fab_R]

Keep in mind now that you've done that, that your entire trip meter is now wrong, including fuel consumption and Odometer. The car KNOWS that it's doing 93/94km/h, but tells you that you're doing 100km/h because it's required to do that by law. If you were to do 100km/h and then reset your trip meter, it would then tell you CORRECTLY that you're averaging 93/94km/h. Now that you've changed those figures, it'll give you incorrect information, incorrect fuel consumption, incorrect range til empty, etc etc.

Good info Corey...

Stevie, I think you may want to change it back man?

[SteveK]

Keep in mind now that you've done that, that your entire trip meter is now wrong, including fuel consumption and Odometer. The car KNOWS that it's doing 93/94km/h, but tells you that you're doing 100km/h because it's required to do that by law. If you were to do 100km/h and then reset your trip meter, it would then tell you CORRECTLY that you're averaging 93/94km/h. Now that you've changed those figures, it'll give you incorrect information, incorrect fuel consumption, incorrect range til empty, etc etc.

Are you sure about that Corey? Not having a go, just trying to understand your thinking. I've worked with guys to do tunes on various local cars and adjusted speedos via electronics to correct deficiencies and my understand is that you are just adjusting the reading from the rolling tyre to the speedo. From there the CPU will use that data to correctly calculate average speeds and fuel usage etc. It's all based on data lines and if the speedo data is incorrect, from the wheels, not what is displayed, then YES the rest of your data is wrong. When compared to various other GPS based speedometers the car is definitely out and after the correction it is closer. I still don't see how that is bad?

Cheers,
Steve

[AdamD]

When compared to various other GPS based speedometers the car is definitely out and after the correction it is closer. I still don't see how that is bad?

I'm not speaking for Corey here, but I believe the point he's making is that your car's speedometer is actually accurate, with respect to the speed the car believes you're travelling. However, a degree of error is introduced into the instrumentation, to ensure the car is road-legal (and this error value is different for the analogue and digital speed readouts). Distance-based systems (so, odometer, trip meter, and fuel consumption based on distance travelled) all rely on the actual road speed - not the number you see on the dial.

The settings you're adjusting with VCDS are presumably there to make allowances for different fitment wheels and tyres, to ensure the car itself gets the speed right. By changing these, you're not actually reducing the amount of error in the instrumentation, but rather throwing off the car's perception of true speed.

[SteveK]

I'm not speaking for Corey here, but I believe the point he's making is that your car's speedometer is actually accurate, with respect to the speed the car believes you're travelling. However, a degree of error is introduced into the instrumentation, to ensure the car is road-legal (and this error value is actually different for the analogue and digital speed readouts). Distance-based calculations based on a measurement of vehicle speed (so, odometer, trip meter, and therefore also fuel consumption based on distance travelled) are all based on the actual road speed.

I think I understand what is being said but I think the interpretation is being lost on me.

The settings you're adjusting with VCDS are presumably there to make allowances for different fitment wheels and tyres, to ensure the car itself gets the speed right. By changing these, you're not actually reducing the amount of error in the instrumentation, but rather throwing off the car's perception of true speed.

There is a contradiction here. You state that changing the VCDS to account for different tyre sizes is what the feature is for but by doing so you change perceived speed of the car.

This is my point exactly. Changing the wheels/tyres will have a physical effect on the perceived speed (what is displayed on the speedo). So you adjust the VCDS to counter for that. Is that not the same as saying "The perceived speed of the speedo is not accurate so you adjust the CVDS to counter for that".

I understand that all cars are created with a set 'speed' rating so that by using the correct size tyres and gear ratios etc, the correct speed will be displayed on the cars dashboard. I know that is what is being mentioned here and I totally agree that this is the case. The point I'm stating is that mathematically, if the dashboard is showing 100kmh on the analog needle and 100kmh on the digital display but is showing 94/95kmh on an external metering device then there is a multiplication factor introduced somewhere along the line that has affected the display.

I've "assumed" that this "Distance Impulse Number" is the 'correction factor' that adjusts this. But if my understanding of the responses in this thread is correct, does this DIN value affect the 'VW agreed value of speed' for the car? If that is so, then I am mistaken. And if so, how is any correction made? Having a car going slower than what is being shown is not what I am used to. My previous car was tack perfect, 100kmh was 100kmh was 100kmh. Every external measuring device was spot on. Overhead speedo checkers, dead on.

Thanks again for helping me understand this.

Cheers,
Steve

[AdamD]

There is a contradiction here. You state that changing the VCDS to account for different tyre sizes is what the feature is for but by doing so you change perceived speed of the car.

This is my point exactly. Changing the wheels/tyres will have a physical effect on the perceived speed (what is displayed on the speedo). So you adjust the VCDS to counter for that. Is that not the same as saying "The perceived speed of the speedo is not accurate so you adjust the CVDS to counter for that".

So, think of it this way: if you have standard wheels and tyres on your car, then, all else being equal, your car knows how fast it's travelling, exactly. That accurate perception of the speed is used to calculate the distance travelled and everything else based on that - so, if you drive at precisely 100km/h for 1 hour, your trip meter will read 100km.

However, your car lies to you about how fast you're travelling. It knows you're doing 100km/h. But it tells you (via speedo) that you're doing 105km/h (or 107, or even 110), so you'll slow down, and stay under the limit. The only error in this whole equation (and it's an intentional one) is in the instrument you're reading. Australian law requires that speedos must read 100% accurate or overestimate up to 10%, but must not underestimate.

What you want is to have an instrument that shows you a (more) accurate representation of how fast you're actually going. So you use VCDS to alter the car's settings to achieve this. Trouble is, rather than having reduced the error margin in the instrument itself (the amount by which the car is lying to you), you're actually making the car think it's going slower than it really is. The nett result is that, yes, the instrument will read a lower speed, but the odometer/etc will underestimate how far you've driven - throwing out its calculations.

I've "assumed" that this "Distance Impulse Number" is the 'correction factor' that adjusts this. But if my understanding of the responses in this thread is correct, does this DIN value affect the 'VW agreed value of speed' for the car? If that is so, then I am mistaken. And if so, how is any correction made? Having a car going slower than what is being shown is not what I am used to. My previous car was tack perfect, 100kmh was 100kmh was 100kmh. Every external measuring device was spot on. Overhead speedo checkers, dead on.

So, I believe what Corey is saying is that the Distance Impulse Number (DIN) isn't the fudge factor in the instrument (the gauge you read), but is rather used to affect the car's understanding of the road speed - to allow for larger wheels, worn tyres, etc. I don't believe we fully understand the relationship between the DIN and the road speed measurement, or the DIN and the instrument-indicated speed, but AFAIK it relates more closely to the former than the latter. If it relates to the former at all, it will have a flow-on effect on the latter. User Logger has done some extensive experimentation with this - might be worth searching out his posts to see what he concluded. From memory, there's no hard and fast way to get a 100% accurate reading out of the speedo, and any adjustment to the DIN can potentially have an adverse impact on your distance measurement.

My suggestion: use an accurate GPS to calculate the error in your instrumentation (digital and analogue) at various speeds. I know in my car, for instance, that 60km/h reads as 63km/h, and 80km/h shows as 85. I got used to sticking to a particular indicated speed pretty quickly. If you hate that idea, keep a portable GPS in the car, and rely on that for a satellite-verified speed reading.

Thanks again for helping me understand this.

No worries - hope it's making sense!