Grades of gas


I have a 1999 Cadillac sevile with a northstar v-8, the owners manual calls for the medium grade of gas 89 octane, this car is used for local driving and a little interstate use, would using 87 octane gas have a harmful effect on the engine. The newer cars with the same engine use regular gas. Fred


A particular engine can be tuned for a particular grade of fuel. It has something to do with ignition timing, valve timing, and so on. Continue using 89 octane as specified. You can get by with 87 octane now and then but taint worth it. Performance will be affected. You spent thou$and$ extra on a Caddy; don’t begrudge it a few dimes on its diet.


If the engine is DESIGNED for a higher octane gas then using a lower octane will result is less performance and gas mileage. So it probably won’t be worth it…especially when you raise the risk for engine damage.


The dime difference is not going to hurt your wallet much. If you drive 15000 miles year your talking a meager $70/year savings. Hardly worth it…


Any $$ you might save by using lower octane fuel will be lost by the reduction in gas mileage, which the computer could command to allow the car to run the lower octane fuel. You might actually wind up losing money in the long run.


Very true. If the difference between 87 and 89 octane is 10 cents/gallon, and your car typically gets around 20 mpg, then all it takes is a drop in mileage of only 1/2 mpg to eat up all your savings at the pump. Any drop bigger than 1/2 mpg means it will cost you most to use 87 octane. This math is valid for gas costing around $4/gallon and the mpg numbers I used. Do the math.


owners manual calls for the medium grade of gas 89 octane

Does it say it is required or recommended. Required means it can damage the engine if you don’t provide the right fuel. Recommended means it can adjust so there should be no engine damage, but it will reduce power and mileage, so why bother??? Some or all you may save on fuel cost will be lost because you will need to buy more.