[deliriousga]

I tried it for two tanks, one running 87 and one running 89. Since my computer has not been chipped, it does as your mechanic said and adjusts the engine to compensate. And compensate it does. Lost power and fuel economy, especially with the 87. I didn't run it for months on either because the performance difference was not worth $1.50 per fill-up.

Btw, if yours is chipped you most likely cannot do this. The computer has built in compensation as mentioned above and the chips fine-tune the engine to run at peak performance so it can't handle lower octane fuels.