Anyone ever notice there speedometer is off?

[rs4 tony]

Anyone experience this?

[dparm]

It becomes increasingly inaccurate as you go faster. From what I have seen on various GPS units, it's a few mph off above 120mph.

[Turbwhistle]

Slower or faster?

[rs4 tony]

I know when I do 80mph on the speedo, my friends car next to me does 75. Both traveling at the same speed. Weird. Maybe because my rolling diameter of my tires throughs it off? 275/30/19's

[rs4 tony]

Slower or faster?

Well I am 5mph slower then the speedo when it's above 50mph. For me to do 40 mph on local roads I have to do 43 on the speedometer or people pass me for going to slow.

[audi49]

speedo says 60. actual is 58 for me.

[rs4 tony]

Any way to fix this issue? Recalibrate it?

[B6JoeS4]

Yeah my speedo is a few mph optimistic.

[Audeeznuts]

Tires size should make a difference. Not sure if it would be noticable enough since its probably not a drastic increase/decrease in size.

[rs4 tony]

Yea true it's a slight slight decrease in rolling diameter.

[Petes4]

At about 75mph my speedo is about 6 mph optimistic.....the dealer said that "they had never seen this before and that they didnt know what was wrong".

[rs4 tony]

I hate problems that come back unanswered. HATE

[zmon]

Tony take a look here. I remembered reading about this, and just found this... Some reading in all of the posts

http://forums.quattroworld.com/rs4b7/msgs/36280.phtml

[rs4 tony]

Good find. Seems like its a common problem in the rs4's.

[ringsroc]

It was a well known problem with the MKV VW Jetta I had. Several reasons givin.

http://tech.bentleypublishers.com/th...rt=0&tstart=40
Here is a thread from the Bentley Publishers site that discussed it fairly well.
I am assuming that Audi and VW share the same philosophy.

[seanj130]

Keeep it, you think your doing 80mph on the highway with your buddies.. they think you are badass, but you are really doing 75mph... Only 10 over and won't be bothered by state troopers shooting Lidar..

[rs4 tony]

Keeep it, you think your doing 80mph on the highway with your buddies.. they think you are badass, but you are really doing 75mph... Only 10 over and won't be bothered by state troopers shooting Lidar..

Lmao. Bad ass. Look at me! =P

[2012S4Guy]

the speedometer's accuracy is determined by a pre-set fixed setting based on wheel diameter. It measures how many times the axle is rotating - or the revolutions per second (Hz).

Ok - some math...

We'll assume the car is set for a 17" diameter tire.

60mph = 1,056 inches/second

Circumference of 17" wheel = 53.38 inches
Revolutions per Second = 1,056 inches/second * (1/53.38 inches) = 19.78 Hz - This is the setting on the computer. So if the axle is spinning at 19.78 times per second, it will register on the speedometer at 60mph.

Now, if you put 18" tires on that vehicle, it is still checking for the Hz of the axle (19.78), however, since it's a bigger tire, it travels further each revolution which means you'll be going faster at the same constant 19.78 revolutions per second.

Circumference of a 18" wheel = 56.52 inches
Actual Speed at pre-set Rev/Sec = 56.52 inches * 19.78 rev/sec = 1117.9 inches/sec = 63.5mph

For a 19" tire:
C = 59.66 inches
Actual speed = 67mph

===============

Long story short, the factory determines the pre-set constant and as you (the customer) change tire sizes, it makes the speedometer readings inaccurate. If the factory sets a smaller diameter than the actual tire size, you'll be going faster than what is shown on the speedometer. If the factory sets a larger diameter than the actual tire size, you'll be going slower than what is shown on the speedometer.

[zachf88]

A lot of vehicles have around a 10% speed inaccuracy. In the US, speedo's read 5-10% higher than they actually should. This is a US "safety" feature. I had an article on this, but i've lost it when my last comp crashed. Many of the radar readouts are also inaccurate by about 5-10% anyway. Also, This can be lessened or strengthened by changing tire/wheel size. GPS seems like the best way to monitor real speed, besides knowing actual mileage and dividing it by the time.

[rs4 tony]

I see. I see. I see it as a good thing tho. Says 80 I'm doing 75* ish. No biggie

[RAudi Driver]

It's "Their" not "There". You guys are an embarrassment to the US education system.

[B6JoeS4]

It's "Their" not "There". You guys are an embarrassment to the US education system.

It's U.S.

[mbgt72]

the speedometer's accuracy is determined by a pre-set fixed setting based on wheel diameter. It measures how many times the axle is rotating - or the revolutions per second (Hz).

Ok - some math...

We'll assume the car is set for a 17" diameter tire.

60mph = 1,056 inches/second

Circumference of 17" wheel = 53.38 inches
Revolutions per Second = 1,056 inches/second * (1/53.38 inches) = 19.78 Hz - This is the setting on the computer. So if the axle is spinning at 19.78 times per second, it will register on the speedometer at 60mph.

Now, if you put 18" tires on that vehicle, it is still checking for the Hz of the axle (19.78), however, since it's a bigger tire, it travels further each revolution which means you'll be going faster at the same constant 19.78 revolutions per second.

Circumference of a 18" wheel = 56.52 inches
Actual Speed at pre-set Rev/Sec = 56.52 inches * 19.78 rev/sec = 1117.9 inches/sec = 63.5mph

For a 19" tire:
C = 59.66 inches
Actual speed = 67mph

===============

Long story short, the factory determines the pre-set constant and as you (the customer) change tire sizes, it makes the speedometer readings inaccurate. If the factory sets a smaller diameter than the actual tire size, you'll be going faster than what is shown on the speedometer. If the factory sets a larger diameter than the actual tire size, you'll be going slower than what is shown on the speedometer.

Yes, I remember seeing a lot of B7 RS4's from the factory with 17" TIRES back in the day. . . .


Sounds like we have another AurtherPE up in here!

[S4Per]

Just spoke to my dealer about this last week. He says they discussed with Audi several years back; Audi wants to make sure that no if's and's or but's, you'll never be going faster than the speedo tells you. There is no way to have this adjusted (mine's off by about 5mph).

[RAudi Driver]

It's U.S.

Thanks, I would have never known to place a period after the "U" and the "S" to denote that I was trying to abbreviate United States.

[WinterRunner]

Yes, I remember seeing a lot of B7 RS4's from the factory with 17" TIRES back in the day. . . .


Sounds like we have another AurtherPE up in here!

I was thinking the exact same thing.

[rs4 tony]

I was thinking the exact same thing.

17's fit over the rotors/calipers?!?!

[PetrolDave]

Speedos overreading isn't a problem - it's a legal requirement!

An E.E.C. decree of 25 June 1976, regulates the speed displayed by the speedometer in relation to the actual speed travelled.

This decree stipulates:

- The speed indicated by a speedometer must never be lower than the actual vehicle speed.
- Between the speed displayed «SD» and the speed travelled «ST», there must always be the following relationship:

ST < SD < 1.10 ST + 4 Kph

For Example: For an actual speed of 100 Kph the speed displayed by the speedometer may be between 100 and 114 Kph.

[2012S4Guy]

Yes, I remember seeing a lot of B7 RS4's from the factory with 17" TIRES back in the day. . . .

Sounds like we have another AurtherPE up in here!

I was thinking the exact same thing.

haha wow, tough crowd. i was just giving a simple example of how 1-2" could give a noticeable change! don't get your panties all in a bunch over it!

[NY07RS4]

Every vehicle I have ever owned has had a speedometer reading higher than the actual speed. When GPS units became common I started doing comparisons for fun. Motorcycle, car, SUV, quad - all read higher than actual speed according to GPS. The boat I had was the worst of the bunch, it was way off.

[A4A4A4]

haha wow, tough crowd. i was just giving a simple example of how 1-2" could give a noticeable change! don't get your panties all in a bunch over it!

1-2" change in wheel size does not equal 1-2" increase in rolling diameter, it's the choice in tire size that makes that difference.

You can have a 20" wheel that has a smaller rolling diameter smaller than a 15" wheel, it all depends on sidewall size.

[audikid66]

I proved this due to the fact I'm pretty sure I topped out my car my Speedo said 160 and I'm pretty sure our cars are stock capped at 155. Correct me if i'm wrong

[jareds941]

I proved this due to the fact I'm pretty sure I topped out my car my Speedo said 160 and I'm pretty sure our cars are stock capped at 155. Correct me if i'm wrong

Oh that proves it fo sho!

[rs4 tony]

I proved this due to the fact I'm pretty sure I topped out my car my Speedo said 160 and I'm pretty sure our cars are stock capped at 155. Correct me if i'm wrong

That's what another rs4 member told me

[BenSti]

I checked my car against GPS, at 192mph on speedo, GPS said 188mph, pretty damn good in my book...

If you access engineering mode on the RNS-E, it has a page to view actual GPS speed, it also has another page which seems to be calibrating the tire diameter by comparing the wheel speed sensors with GPS speed. I know many other cars actively calibrate the speedometer, so it is possible ours does it too.

[VMRWheels]

What year is your car?

[rs4 tony]

Mine is a 07. How do u access the engineering screen?

[VMRWheels]

The tires you're running now has a 2.04% diameter difference from your stock tires, meaning that your speedometer will appear 2.089% MPH too fast. For example, if you're driving at 60 MPH, your speedometer will read 61.2 MPH.

[BenSti]

I forget the exact key stroke needed, I think you press the Nav button, hold down the upper left button by the knob for 5 seconds, release and hold down the upper right until the screen changes. It takes a couple tries to get perfected.

[rs4 tony]

The tires you're running now has a 2.04% diameter difference from your stock tires, meaning that your speedometer will appear 2.089% MPH too fast. For example, if you're driving at 60 MPH, your speedometer will read 61.2 MPH.

Hmm not sure about that one. I was doing 65 and my buddy next to me did 60