Ok. I have this problem like this. Could just be conincidental that it's happened when you changed the tyres.
This is the problem I have with my hicas. If I drive on the M1 (Brisbane - Gold Coast) to about the Strawberry Farm half way down the coast (say 30/40ks?) the light comes on.
I've had heaps of problems with my hicas since purchasing the car. Once it was realised that there were wires disconnected, it was all fixed. Then driving up to the sunny coast one night the light came back on. I was fuming.
Ended up getting the car put onto the diagnostics machine and it said that there was a fault in the sensor in the steering wheel.
Basically to put it simply, there is a sensor in the steering wheel that can loose contact sometimes. This happens if the wheel is held straight or moved slightly for a long period. I was told this sensor was about $300 to fix but seeming it only happens on the highway (when hicas comes into effect) I didn't bother getting it fixed because I dont personally do highspeed lane changes and drastic steering movements on a straight road.
If you can, try and get someone to plug your car into a consult machine and get a report printed out on what fault is coming up in the diagnostics.