Thanks for the response. I don't drive beyond New England often enough to get meaningful information on other regions. Seasonal variations are something I am tracking but didn't mention. Summer mileage is higher than winter mileage, not surprisingly. I don't have enough data yet to break it down further.

Mobil was a surprise. It was my go-to brand for 20 years on previous cars, but I never really compared it in a systematic way until now. I will probably buy it occasionally in the future and continue to track. Maybe there's a bias in my data that will average away over time.

The two things I can say with a bit of confidence: octane is not a significant factor in fuel economy; bargain no-name brands are not a bargain.