As the other respondants have indicated, if you don't want to get too technical, it is safer to stay with premium fuel. For myself, I don't, as I have a keen ear for engine knock. The iso octane difference does not change the power or energy output or mileage between regular and high test, unless there is a knock. And knocking or pre-ignition, a shock wave, can be brought on by high engine temperatures, high ambient temperature, changes in ambient humidity, ignition timing, carbon buildup in your engine, etc. I giv emy engine about as much timing advance as she'll take, then back it off a few degrees, and drive with regular, getting about 17 MPG and great performance. Mixture is biased rich as too lean is far worse. With Suzy's car, she should stay safe with high test, but I would check her car's rear end ratio, and get a printout of the exhaust emissions and track ignition on a scope. Her car may have bad points or wrong dwell, or if electronic ignition, have some peak secondary voltage problem (wrong coil for electronic ignition?). Overly rich mixture will sure kill mileage, and needs a real expert to adjust.
Regards,
Joe Bango
