How can I determine if it is more cost effective to run premium fuel in my car?

Question

Every Time I fuel up my car, I collect my odometer reading, fuel price, and gallons pumped. A while ago I drove my car to almost empty and put 93 octane fuel in my tank, ran it to almost empty and did the same thing with 89 fuel and 87. As predicted, the 93 had the best fuel economy, 89 second best and 87 was the lowest. What could I do to determine which is more cost effective?

Answer 1

The simple fact is, it's all just mathematics. For instance:

(Note: I'm just throwing numbers out here, it's the mathematics which are important.)

Given: 20 gallon fuel tank

If you are using 93 octane fuel, get 20mpg, and it costs $2.40/gallon

At 20 gal and 20mpg, that's a total of 400 miles travel distance. 20 gals of fuel at $2.40/gal, would equate to $48 for the tank. $48/400=$.12/mile

If you are using 87 octane fuel, get 18mpg, and it costs $2.00/gallon

At 20 gal and 18mpg, that's a total of 360 miles travel distance. 20 gals of fuel at $2.00/gal, would equate to $40 for the tank. $40/360=$.11/mile

In this instance, the lower octane fuel would produce a better cost to run than premium.

With that said:

As predicted, the 93 performed the best, 89 second best and 87 was the lowest.

I'd be surprised if this is actually true unless your car specifies for the higher octane fuel. The rule of thumb states, use the lowest octane fuel you can run without knocking. You'll usually get better fuel mileage by doing so. Do not equate octane to fuel mileage, because doing so is a false premise. It isn't just me who states this ... you can find it everywhere you loo:

FuelEconomy.gov states:

It depends. For most vehicles, higher octane fuel may improve performance and gas mileage and reduce carbon dioxide (CO2) emissions by a few percent during severe duty operation, such as towing a trailer or carrying heavy loads, especially in hot weather. However, under normal driving conditions, you may get little to no benefit. (emphasis mine)

NICOClub.com states:

Time and again, people will be told that high octane fuel burns cleaner or more completely, and that it will give them extra power and better fuel mileage than Regular octane gasoline because it contains more ‘energy.’ These blanket statements are simply not true. In fact, the octane rating for gasoline has nothing to do with the amount of power locked inside of it – it actually relates to just how much a fuel can be compressed before igniting. The higher the number, the less likely it is to ignite under pressure.

CarsDirect.com states:

Not only will the octane level not effect your gas mileage, but it does not even increase the amount of power your car puts out.

(PS: If you want to see more information, do a Google search )

If your car does not specify the higher octane and you truly are getting the better gas mileage with 93 than with 87, it may be that in order to truly get honest numbers, you'd need to run more than one tank of each and utilize the final tank full as your base number for each octane level. The reason for this is, it take some adjustment time for the computer to actually run the engine at its best. By not giving it a chance to optimize for the fuel being used. You could speed up the process by pulling the battery cable for 30 minutes (or the PCM fuse, if so equipped) to allow the PCM to reset. This would bring the fuel map back to factory stock. This would get you your truest MPG figures in your experiment.