I checked out the cluster with the VCDS and it was coded by default to 3.
First, I used a measuring tape and a piece of wood laying across the top to measure the tire's diameter. I got 24 7/16" which is roughly 620mm. According to a tire size calculator, the diameter of my tire should be 632mm, but I'm pretty sure that's for when there's no air in it or weight on it, which is never the case when it's mounted on the car. So for now I'll use my measured diameter of 620mm since I feel it's more accurate.
I set up my laptop with VCDS and, using my TomTom GPS as a guide, I brought the car up to 100km/h and set the cruise. The GPS reported a steady 100km/h each time I tested a new multiplier. I know some of you may have certain opinions about the accuracy of GPS, but it was the best available tool I had to objectively measure my speed. If you don't agree, tough.
The results from watching the
speedo needle when the GPS reported 100km/h are as follows (1-7 are the coding digits):
- Multiplier = 22188 : Underreports by ~3-4%
- Multiplier = 22076 : Underreports by ~2-3%
- Multiplier = 21960 : Underreports by ~2%
- Multiplier = 21848 : Overreports by ~5-7%
- Multiplier = 22304 : Underreports by ~4-5%
- Multiplier = 22420 : Underreports by ~6-7%
- Multiplier = 22532 : Underreports by ~7-8%
Those numbers are just ballpark figures based on what I remember.
Now, the fun part. I decided to watch the cluster's measuring blocks and watched 001 while driving. On multiplier #3 (21960, factory default) the measuring blocks report the car going exactly 100km/h. I sped up and slowed down and the measuring blocks were identical to the GPS for speed. Based on that, multiplier 3 is exactly what we should have for coding on these cars with 205/55R16 tires.
Going back to my calculations from an earlier post, I posited that the multipliers are the result of multiplying 48 by the number of tire rotations in 1km. I forget where I got 48 from, but it was online somewhere. Using the ideal multiplier that I derived from that assumption, I got 24643, which is about 2000 higher than the closest available coding choice. This didn't make any sense to me. How could it be that far off, yet #3 is what they set at the factory? I decided to look up a picture of an ABS ring online. I looked at one for a 2004 Jetta (site didn't have a listing for ABS rings for A5 models), so I'm assuming is still the same part. Either way, when I counted, there were only 43 slots/ribs, so 43 impulses per tire rev! Dividing the distance impulse multipliers offered as coding choices by 43 will yield exactly how many tire revs/km the multipliers are coding for!
- Multiplier = 22188 : Coded tire revs/km = 516
- Multiplier = 22076 : Coded tire revs/km = 513
- Multiplier = 21960 : Coded tire revs/km = 511
- Multiplier = 21848 : Coded tire revs/km = 508
- Multiplier = 22304 : Coded tire revs/km = 519
- Multiplier = 22420 : Coded tire revs/km = 521
- Multiplier = 22532 : Coded tire revs/km = 524
With that mystery solved, the issue still remains that the needle isn't pointing in exactly the right direction. From what I can tell, the needle is underreporting by about 2km/h at 100km/h and would likely just take a small turn clockwise to bring it in to where it's 100% accurate!
I'm bushed and can't think of anything else, so if you've got anything else to add, please do so!
Cam