How do I safely change the grade of gas I use from 91 to 89?

I have a 2000 corolla and have always used 91 because it is ';recommended'; in the manual.How do I safely change the grade of gas I use from 91 to 89?
The main reason cars use higheroctance fuel is because they have a higher compression. if a low octane fuel is used in a high compression vehicle you will get combustion before your spark plug even sparks (also known as detonation) Alcohol wil also help prevent detonation as it raises the octane level of the fuel. If you want to use a lower octane fuel do it gradually. try mixing mid-grade with premium and run that a while and then maybe to just mid-grade and so on. but if your can starts to ping then do not lower the octane level. BUT i am not sure if your car has a high compression engine or not so but if that is the case then you could eventually bring it down to 89 octane.How do I safely change the grade of gas I use from 91 to 89?
Don't worry about it... just fill up with whatever you'd like. There is nothing unsafe about doing this. It won't hurt your car.



Try keeping track of you gas mileage and see which tankfull gives you the most miles per gallon.



My guess is, you will get ONE MORE mile per gallon with the higher octane gasoline.
just put it in and see...if it pings and runs like a cement mixer change back!! really compuiter will adjust it to run on 89...you will get less power and lower mpg but probily not enough differnt to know..
you cna change it when ever you like what you want to do is watch out with the Ethonal. Its against the law to not post the percentage on the pump and it has to be shown where people can see it. I use gas with no ethonal. You can also add injector cleaner ever few weeks. The best one is from Lucas and you get it at most parts stores. The reasons they say what grade to use is for the octane and you can get a bottle of it at the gas station as well the octane keeps the injectors clean and the Lucas helps with cleaning them its what I use.
it's recommended because it will give you the most MPG (miles per gallon) you can use any kind you want but if you think the qualaty is too poor change back or if you are expirenceing problems with your car.
will the best to change over is 2 run the car close to empty then put the 89 in it . if your car runs bad u can set the timeing and put some gas boost in it .if that don't help u wil have to change back to 91
fill up with 89 and add an octane boost, then every time you fill up after that add less octane boost until you don't need it at all.
Always use the fuel recommended by the manufacturer. Using lower octane fuel will cause knocking and form soot inside the engine -- which will result in lower mileage and will end up costing you more.



However, I have a 2000 Camry, its manual recommends 87 octane - kind of surprised to hear that corolla recommends 91.