I'm starting to get less gas mileage...?

[pdiggitydogg]

youre not going to get better mileage out of higher octane
switch to 87
The standard civic is designed to use the lower rating (because it doesnt need to use higher)...its the proper fuel.
Youre just wasting money dude.

Octane myths

High octane gasoline improves mileage.
In general, if your car is designed to run on 87 octane gasoline, high octane gasoline will not improve mileage. If switching to high octane gasoline does improve mileage, you might find that a tune-up will give you the same improvement on 87 octane gasoline.

High octane gasoline gives quicker starting.
No, it doesn't.

High octane gasoline increases power.
If your car is designed to run on 87 octane gasoline, you shouldn't notice any more power on high octane gasoline. Again, if it does make a noticeable difference, you may need a tune-up.