Change sizes from 215/45-17 to 205/50-17 ?

[rpn453]

Originally Posted By: Quattro Pete
Sometimes this is used as a poor-man's tire pressure monitoring system. For example, in my 530i, if my tires are significantly underinflated, my ABS light will be flashing when I drive.

ABS has wheel speed sensors. I don't know how exactly this works, but possibly the car's ECU expects a certain RPM to wheel speed ratio. If that ratio falls outside of a preset range (as a result of underinflation or using a tire size that's notably taller/shorter than OEM), then ABS will complain.


Audi still uses that system for detecting underinflated tires. Good system; no extra sensors to deal with. But it can only see differences between wheels, not a change in all four. Everything from the engine to the wheels will increase or decrease in speed proportionally to the tire change. The only way I can think of that it would know of a change in tire size is if it compares to GPS-based calculations or has a radar/laser system that can directly measure speed.

 

[Quattro Pete]

Quote:
But it can only see differences between wheels, not a change in all four.


This article talks about it, but it doesn't go as far as explaining how a different tire size (on all 4 corners) upsets the ABS...


Quote:
Changing tire sizes will change the WSS inputs, which may upset the operation of the ABS, traction control or stability control systems. On some vehicles, the control module can be reflashed to accommodate a change in tire size. But on most vehicles this is not possible. So the vehicle owner has to stick with the same OEM tire and wheel size. If they want to replace their OEM wheels and tires with aftermarket wheels and tires, they can go with plus wheel sizing and lower profile tires to keep the overall tire diameter the same as before.


http://www.brakeandfrontend.com/Article/46336/diagnosing_wheel_speed_sensors.aspx