hi, i have the haybusa muffler on my is200 too, i find the first week or so... the fuel economy quite good, give alot of power
but after i reset the ECU, i find the fuel economy went up..... i use a TRUST pe2 muffler b4 and i could do around 480km b4 the light comes on..... but after reset ECU with the haybusa muffler could only do around 400.......
and once reset the ECU, i find i loss alot of low end torque, i have now installed the sliencer on the muffler, the car get abit more torque and less noisy and can do around 450km.....
so i think u will find that as ur ECU relearn ur new muffler, it will readjust the fuel ratio and ur fuel consumption will go up by alot