about a year ago started using reg gas in my 2007 lexus (because of gas prices) I don't see any difference -but wondering if I'm hurting the engine in the long run -
The people that know the answer to your question wrote the owners manual. Check there.
I feel your pain but do the math. Will the money save be enough to buy a new engine in the amount of time it takes to die? Are you willing to part with the car (and the bulk of its' value) due to an early death? One tank isn't going to do something you'll notice, but over time you will be compromising the function and condition of your engine.
I occasionally put 89 in my Lincoln but almost always use the 93 or 91, whatever the particular station has. Keeping a good (paid for!) car in good condition is easier and cheaper than beating it up and having expensive repairs.
On high-test in the Lincoln. I didn't realize that any Ford products required 91 or 93.
There's a sticker right inside the gas cap flap that says "premium recommended."
I copied/pasted this info from cartalk.com:
In a high-compression engine that's designed to run on premium fuel, premium will provide some additional power. But, if you want to save some money, you can probably still fill up with regular, unleaded fuel much of the time.
An engine with a high compression ratio has some advantages. It squeezes more power of the same-size engine. But, it requires you to spend more for every tank of gas, to get that extra power. In other words, instead of manufacturing a car with a larger, but lower-compression-ratio engine, manufacturers are using high compression engines, and foisting the additional cost back to you - by forcing you to buy premium gas to get that extra performance.
Using regular gas can cause knocking and pinging. They're noises, which are signs that the gas and air mixture in your car's cylinders isn't burning exactly as intended. The sound you hear is the result of the fuel and air mixture combusting unevenly in the cylinder. Pinging or knocking will reduce the efficiency of your engine and, over a very long period of time, it can cause damage.
They put KNOCK SENSORS in cars to guard against improperly fueled vehicles (timing will change if knock detected). Check your operating manual.