After multiple tanks of gas to test this theory my car gets 2-3mpg better on Shell gas then it does BP and both are regular unleaded in my test. It gets the worse gas mileage using Hess gas. This is watching the tank averages and a very consistent country road commute to work for 35 miles one way with virtually no traffic running 59mph.
Please be careful with these statements. BP and Shell are both top tier. Your mileage should NOT vary. Did you actually track your mileage – fill the tank each time and divide the miles traveled during the last tank by the gallons needed to fill it? Did you do that for tank after tank after tank? If yes, what were your exact numbers?
Also, you must not compare summer fuel mileage to winter fuel mileage. Winter is always worse, even on the same brand of fuel. You must also avoid comparing fuel economy on winter blend fuel to summer blend fuel. The “colder” states just changed from winter to summer fuel last month (May 1st). All summer blend fuel gets better fuel economy than winter blend. If you used BP in April and Shell in May, you might have very well seen an increase in fuel economy, but it had nothing to do with Shell fuel being superior to BP. It had to do with all gas companies changing from winter to summer blend.
Seriously all things equal, one gas will not provide 10% better fuel economy than other. (Unless it is pure gas - no ethanol.) All gas comes from the same refineries. Any additives are added after the fuel leaves the hub. Further, most of those additives are for keeping the fuel system and engine clean – for longevity. They don’t give the car more power or increase fuel economy.
I believe MMT is used to increase octane. But it is super toxic – less toxic than lead but still very bad for us. I believe Honda formally states to avoid gasoline with MMT because it creates deposits in the engine and exhaust system. I also believe it voids the warranty.