Can I just add that ABS systems don't use some set logic (on if "this", off if "that") to decide absolutely when to activate. It is a control system with pretty advanced logic and algorithms to measure some parameters and estimate others using the measurements. The math is pretty complex.
Anyway, the OP's problem has to be caused by the different diameter tires. Mango is saying that the brand is doing it, but I doubt that two brands would be off by more than 1-2 rev/mile. I bought my car with 2 brands on it and it behaves fine. I will get 4 new michilens when I can, but for now, the tires work. OP already admitted he has different diameter tires on front and rear axles. The problem lies there or maybe a wheel speed sensor.
Last edited by WDE46; 10-03-2012 at 02:01 PM.