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17. Should you use low octane fuel?
Maybe. If your owner's manual calls for premium, use premium. If you use regular instead, the cars computer will retard the ignition timing automatically to prevent detonation. This will have a devastating effect on gas mileage, which will more than offset lower fuel price. Detonation occurs only on full or almost full throttle, so you may be safe using lower grade gasoline in this case if you are conservative with your acceleration habits. It all depends on your driving style and how your car's computer handles the changes. It might be worth giving it a try.

18. Do you need to do a tune-up?
Maybe not. You will never recover the cost of a tune-up in fuel savings. However, you should do regular maintenance, not only for gas saving, but also for performance and reliability. Don't go to a garage and buy a tune-up, 10,000 miles before you need it because you think it will help your mileage... it doesn't work that way, sorry.

19. Do you need to switch to synthetic oil?
Maybe. Synthetic oil is great for engines, and does help gas saving a bit, by decreasing parasitic losses in the engine. But it is significantly more expensive than the regular oil, and its gas saving effect is nowhere near a trade off for its price. However, if you are already considering a switch to synthetic oil for any reason, you can surely also count on some gas savings as well.