Speedometer Error

[Heldyballs]

What kind of speedometer error are you guys seeing?

I've had my RS4 for about a year and one of the first things I noticed is a significant difference between the speedometer indicated speed and GPS speed using several apps on my phone (Waze, Speedpro, Speedometer+). In previous cars that I've owned the speedo has been somewhat optimistic, but not like this.

My extremely unscientific data:

Speedo says: Phone says:
30mph 28mph
50mph 47mph
60mph 56mph
70mph 65mph
80mph 76mph
90mph 85mph

First of all, the error seems inconsistent, which is pretty annoying. If the speedo were always 2% off, I could live with that and always bump the speed up a couple MPH, but different speeds seem to invoke different errors. Second, the error seems significant. As low as 5.5%, and as high as 7%. Since the speedometer and odometer get their data from the same place, that means total mileage could be 5-7% off. Maybe it is actually insignificant, but I think its kind of BS that a car that has driven 93,000 miles says that it's gone 100,000.

From what I've read, German auto regulations mandate that a car's speedo cannot underestimate speed, but I can't find a limit to it's overestimation. I can't find anything about US DOT limitations.

Worth mentioning that I'm on stock wheels with stock sized tires (255/35-19), about 6/32nds all around inflated to 42psi.

Any thoughts?

[rollerton]

Yes sir. That' how US cars are from my experience. As as far as I can tell the speed discrepancy doesn't translate to the odometer, but having owned something like 17 modern VW/ Audi cars they ALL generally display that much error.
Some thread a few years ago there was someone who implied that US DOT regulations accounted for the speedometer error and it has to do with the gauge not being allowed to read LOWer than actual speed, and the roughly 3-6% ensured that would never happen if the speedo was ever to not read within specifications to the extent that it was't noticeable by the driver.

[GrandMarkie]

What kind of speedometer error are you guys seeing?

I've had my RS4 for about a year and one of the first things I noticed is a significant difference between the speedometer indicated speed and GPS speed using several apps on my phone (Waze, Speedpro, Speedometer+). In previous cars that I've owned the speedo has been somewhat optimistic, but not like this.

My extremely unscientific data:

Speedo says: Phone says:
30mph 28mph
50mph 47mph
60mph 56mph
70mph 65mph
80mph 76mph
90mph 85mph

First of all, the error seems inconsistent, which is pretty annoying. If the speedo were always 2% off, I could live with that and always bump the speed up a couple MPH, but different speeds seem to invoke different errors. Second, the error seems significant. As low as 5.5%, and as high as 7%. Since the speedometer and odometer get their data from the same place, that means total mileage could be 5-7% off. Maybe it is actually insignificant, but I think its kind of BS that a car that has driven 93,000 miles says that it's gone 100,000.

From what I've read, German auto regulations mandate that a car's speedo cannot underestimate speed, but I can't find a limit to it's overestimation. I can't find anything about US DOT limitations.

Worth mentioning that I'm on stock wheels with stock sized tires (255/35-19), about 6/32nds all around inflated to 42psi.

Any thoughts?

For me, anything above 50ish mph is about 5mph off (speedo reads high)

[WinterRunner]

Speedo always reads a few MPH higher, you can get a GPS MPH read from your RNS-E too.

[lucidmatt]

speedo reads high for me, even with larger cross section tires (255/40/19 winters) but the gap closed to about 2-3 mph higher than GPS reads. I guess it keeps you out of trouble!

ETA: I think there is a speedo calibration function on VCDS? I havent messed with it yet, but has anyone tried that solution?

[rs4max]

Speedo will always read high, unless you have a crazy tire configuration. This is actually required by law upon car manufacturers. There is no regulation on the accuracy though, but it will usually not be more than 10% (again, unless your car is broken in some way ;-). I don't really care, as most of the time I don't care about my accurate speed unless I'm trying to be on cruise control for a long trip, in which case I'll rely on GPS speed.

[FWP!]

Speedo will always read high, unless you have a crazy tire configuration. This is actually required by law upon car manufacturers. There is no regulation on the accuracy though, but it will usually not be more than 10% (again, unless your car is broken in some way ;-). I don't really care, as most of the time I don't care about my accurate speed unless I'm trying to be on cruise control for a long trip, in which case I'll rely on GPS speed.

All of my friend's B7 S4's have speedometers that are dead on accurate. I have brought up this issue before and have thought about changing the tire size in VAG COM, but I didn't figure it out.

[Striddy]

Same here. About 5mph off

[zachf88]

its about 8%. All US spec cars are like this, unless you increase rolling diameter of wheels.

[Heldyballs]

Well I guess I'm happy that it's not just my car although the compulsive side of me still hates that it's significantly off. I personally don't think an error of as much as 8% is acceptable in a car like this where everything else is overengineered, but thats just me. Does anyone know if the odometer gets its info from the speedo or GPS?

[PetrolDave]

Speedo says: Phone says:
30mph 28mph
50mph 47mph
60mph 56mph
70mph 65mph
80mph 76mph
90mph 85mph

Any thoughts?

I don't know if it's the same in the US, but I suspect it's pretty similar throughout all car markets, in Europe it is a legal requirement that the speedo over-reads by between 0 and 10%.

So at an indicated 30mph the actual speed must be between 27 and 30mph, and at an indicated 90mph the actual speed must be between 81 and 90mph.

[WinterRunner]

^ Pretty sure it is.