The vast majority of cars and trucks on the road are designed to run on 87 octane. Once you start increasing the compression ratio in the engine, you need to upgrade to the higher octane ratings. Take my Contour for example... It is a 95 with the 2.5 liter v6. The owner's manual specifically states that anything higher than 87 isn't really necessary. However, when you take into consideration the 140k miles worth of carbon build up on the pistons and in the heads, the car really does benefit from 89. So much so, that there is a clear "seat of the pants" difference between the cheap stuff and the midgrade. Try this, run the current tank almost all the way out (make sure you can get to the gas station though, DUH!). When you get gas, get a half tank of the cheap stuff. Run around on that for a couple weeks to see if you can tell a difference. If there is no loss of power and no pinging or anything, save that extra dime a gallon and buy cheap.