This isn’t a question, just an example of the difference two batches of gas can make on MPG.
Yesterday I went from Wash. DC to Philadelphia PA in my sister-in-law’s Toyota Avelon. Her car has an MPG meter that indicates average MPG and resets every time the car resets. She filled up at a cheap independant station before we left. We went up and half way back averaging 21-22 MPG on the interstate, making several food and rest stops. She drove up and I drove back. We both got virtually the same milage even though I drive a little faster. I thought the car was a little thirsty, but it’s a pretty big sedan with four passengers and a full trunk. Anyway, I stopped about halfway back and refilled the tank with regular 87 octane at a Sunoco. It was down to 1/8th of a tank. When we got back out on the highway the MPG immediately went to 27 MPG and climbed to 29 MPG by the time we got back to DC city limits.
Just goes to show the independent stations sometimes get batches of inferior gas.
While I absolutely agree with your premise, you evidence is faulty. The routes were not the same, so variables other than the gas may have accounted for most or all of the difference you noted.
not to burst your bubble (of hydrogen!) 
but those built in MPG calculators are notorious for inaccuracy. the mileage, divided by the gallons is the only way to accurately figure this out.
did you confirm the electronic to the old miles / gallon method?
Yeah, I agree with NYBo and cappy208. I have not found any significant difference in mileage between different gas stations’ fuel.
You might want to check this however: Did the first station have 10% ethanol added and the second station not? If so this can make a noticeable difference.
I suspect you’ll now be a Sunoco fan for life. If you’re happy, then I’m happy for you.
Different brands of gasoline have never made one iota of difference in any of the cars I’ve owned.
Actually, the routes were very similar, if not exactly the same. We got off the interstate, got gas, and got back on the interstate within 1/2 an hour. Same level road between Wilmington and Baltimore. We did this several times going up and coming about half way back, the average resets each time the car was turned off. Even with different drivers the mileage stayed 21-22. Only when we got gas the numbers went up immediately.
I don’t know what station my sister-in-law filled up at initially.
I should have followed my better judgment and not mentioned any name brands. Sunoco just happened to be the brand at that particular rest stop. I have in the past gotten the occasional low mileage tank of gas from an independent station. They don’t sell a consistent “brand.”
There are several variables that can make a difference in gasoline mileage. A headwind can make a couple of miles per gallon difference. As others have said, the MPG meters aren’t always real accurate. Years ago, oil companies advertised different additives in their gasolines that would improve mileage. Shell had TCP and later platformate. D-X had boron. Standard Oil had a final filter to keep foreign matter out of the fuel system. In those days, I couldn’t tell any difference in gasoline mileage no matter what I put in the tank. In my area, the same distributor sends its trucks to different gasoline stations. I understand that there may be an additive package that is added by certain labels, and I think Sunoco may add a blue dye to its products, but I’ll bet it doesn’t make any difference in the gasoline mileage.
I agree.
The only way to accurately calculate gas mileage is by refilling a tank, noting the elapsed number of miles since the previous fill-up, and doing the usual calculations. Basing gas mileage on a dashboard-mounted mpg device, a gas gauge, or anything other than refilling a tank and doing the proper calculations, will inevitably lead to conclusions that are not correct.
While I always use name-brand gas, I have never found that different brands of gas yield different mpg figures under the same conditions.
You realize that premium fuel is recommended for that car right? Remind me never to let you borrow my car. Seriously though, you experiment has to be taken with a grain of salt. Since you are going more uphill one way and more downhill the other. You also said you drive faster, so that will throw the measurements out of whack too.
Like I said: we got off the highway, got gas, got right back on the highway. There was no significant change in wind. There was no difference traveling North vs South, until we got more gas.
“You realize that premium fuel is recommended for that car right?”
No. I asked my sister-in-law what to put in and she said regular. It’s her car and I didn’t second guess her.
There was no difference in mileage with her driving vs me, I wrote that in the original post.
It seems you didn’t read the OP very carefully.
" While I always use name-brand gas, I have never found that different brands of gas yield different mpg figures under the same conditions."
My assertion regards name-brand vs independent stations, which get their gas from different sources.
Don’t worry, it won’t wreck the car. The computer will retard the timing if it is recommended, if it is requires it can cause knocking and more problems down the road. I did think that regular was what was called for with the Avalon, but I may be wrong and don’t want to second guess FoDaddy.
I’m just stating that since you did not take the exact same route the results are not definitive. If you had taken results from the same drive it would be accurate. By that I mean that you would take measurements of the same trip (envision filling up on the trip from DC to Philly at one station, driving to Philly then recording what mileage you got. Then coming back to DC, disregarding the mileage since it’s not the same trip. Then once you get back to DC, fill up at another station then drive to Philly again using the same exact route and seeing what the mileage is one that trip.) Obviously that’s not a practical way to determine if a certain station is selling bad gas. But it would be more accurate. For example I can’t really compare mileage on trips where I drive from Richmond to Blacksburg to see at Va. Tech game, because the trip to Blacksburg is almost all uphill, and the trip back to Richmond is downhill. And for the record, the 1MZ-FE engine in the Avalon was made to operate on 91 octane. You can use 87, but you’ll be giving up about 15 HP and about 3 MPG.
Are you sure the MPG computer automatically resets? In my experiance, you usually have to manually reset them. If the MPG computer had a hundred miles or so of mostly city driving on it when you left, the average would start out at around 21-22, which is probably the expected city mileage, but as you did more miles the average would go up closer to the 30mpg or so that should be the highway mileage on this thing. This would be consistent with your observations.
As the other posters have noted, the miles driven divided by gallons needed to refill method is the only accurate method of determining gas mileage.
“My assertion regards name-brand vs independent stations, which get their gas from different sources.”
And, those different sources are actually the the name-brands, albeit with an unknown pedigree. (You didn’t really think that “Sonic” gas, or “Astro” gas, or “Sav-on” gas have their own refineries, did you?)
Ergo, “different” brands of gasoline includes the independent stations, since their gas comes from the name-brand companies.
Hey, I just checked a map. DC to Philly is all uphill! The return trip is all downhill! So you got better mileage on the way back?? C’mon… Duh!
“Are you sure the MPG computer automatically resets?”
Yes I am. Every time the car was restarted the MPG figure started out widely differrent then settled in to a pretty steady number. For instance, after we got gas and drove through the rest area and down the ramp onto the highway the MPG registered about 8 MPG. Then after a few miles it rose to 27 MPG.