2014 R-Spec: 16,000 mile gas mileage experiment

[BioSci]

I bought my 2014 R-Spec brand new in April 2015 with 39 miles on the odometer. Being an engineer by training, I started keeping meticulous gas records in a notebook. I started using 89 octane gas when it was brand new and switched to 93 octane at 4,000 miles. Switched back to 89 octane at 8,000 miles and again to 93 octane at 12,000 miles. Here's the results of my gas mileage experiment (calculated by summing up the miles driven with each grade of gas divided by the total gallons of each grade, NOT by averaging each tankful). I drive in mixed city and highway conditions in the Boston area, probably 60% city and 40% highway (considering rush-hour highway miles driven at speeds under 40 MPH).

93 octane: 19.4 MPG
89 octane: 19.0 MPG.

Conclusion: slightly better mileage at 93 octane, but not enough to make up the price difference. I think for now I'll stick with the 89 octane. However, I also conclude that this car gets awesome "smileage" (smiles per gallon).

Mike in Mass

 

[Audio]

Being from Maine, I want to make a snarky "Mass hole" comment, but I dig this post. Now, there are more benefits to higher fuel grades than just fuel mileage, but if mileage is your main concern, that's a damn fine experiment you did there!

 

[Rey]

I did 44K miles on a '12 RSpec using 87 octane almost exclusively. From time to time while traveling I would fill with 91 octane to see if there was a back-to-back difference. Always got better mileage with 87. Had the same result in my '05 Cadillac CTS-V.
Basically, I think the BTU is a bit higher with 87 and as long as ignition is not dialed back because of lower octane, then better mileage. In town driving will cause ignition to dial back sometimes with 87, which degrades gas mileage to less than 91. Whereas on the highway at a more or less steady state cruise ignition timing remains at spec.