Introduction
Modifying tire size on your vehicle can impact speedometer accuracy and odometer readings. While the car’s computer often compensates for minor variations, significant changes require adjustments using diagnostic tools like VCDS (Vag-Com Diagnostic System). This article explores using VCDS for “Vcds Odometer Change” specifically focusing on adjusting tire circumference settings in an Audi Q7 (4L) to maintain accurate readings after switching to a different tire size.
Understanding Tire Circumference and VCDS Coding
The Audi Q7’s onboard computer utilizes data from the ABS sensors to calculate speed and distance. This calculation relies heavily on the tire circumference setting. When tires are changed, especially to a size with a significantly different circumference, this setting needs adjustment using VCDS to ensure accurate speedometer and odometer readings.
The tire circumference setting is typically found within the Instruments module (Module 17) in VCDS. This module contains a coded value that corresponds to a specific tire circumference. However, the relationship between the coding value and the actual circumference is not always linear or documented.
Decoding the Audi Q7’s Tire Circumference Settings
In a specific scenario, an Audi Q7 owner is looking to change from 295/35 R21 tires (circumference of 2324mm) to 255/55 R19 tires (circumference of 2397mm). This represents a 3.1% increase in circumference.
Accessing the Advanced Measuring Values in VCDS confirms the current tire size falls within the acceptable range, while the planned size does not. Further investigation into Module 17 reveals a limited selection of pre-defined tire circumference options:
Friday,23,February,2024,21:44:21:36005 VCDS -- Windows Based VAG/VAS Emulator Running on Windows 10 x64 VCDS Version: 23.11.0.0 (x64) HEX-V2 CB: 0.4643.4 Data version: 20240205 DS351.0 www.Ross-Tech.com ------------------------------------------------------------------------------- Address 17: Instruments (J285) Labels:. 4L0-920-xxx-17.clb Part No SW: 4L0 920 933 H HW: 4L0 920 933 H Component: KOMBIINSTR. H35 0332 Revision: 00000000 Serial number: X2M50177F Coding: B356630400010008010704 Shop #: WSC 31414 790 00001 VCID: 323D0DC50427ABCAE97-8066 No fault code found. -------------------------------------------------------------------------------Initial findings indicate a coding value of 4 corresponds to a measured circumference of 2340mm (actual) with a medium value of 2275mm. The challenge lies in determining the circumference values associated with the remaining seven coding options.
Challenges and Considerations for VCDS Odometer Adjustment
Crucially, the order of coding values may not directly correlate with increasing or decreasing circumference. Experience with other Volkswagen Group vehicles suggests a non-linear relationship. Additionally, certain models may restrict adjustments, allowing only increases in the coded value.
Conclusion
Adjusting tire circumference using VCDS is essential for maintaining accurate speedometer and odometer readings after a tire change. However, the process requires careful consideration of the specific coding values and their corresponding circumferences. Due to the undocumented nature of these values, experimentation and community knowledge are often necessary to determine the optimal setting. Further investigation is needed to identify the appropriate coding value for the desired 2397mm tire circumference in this Audi Q7 example.
