Last fall I traded cars. When I purchased the car the mileage around town showed 22.5 but as I drove it it steadily went down until it was from 17 to 19 Miles per gallon around town and out on the road I couldn’t seem to do better than 26 or 27 although my car was rated to get 28 on the highway. I want to mention that I bought all my gas at Costco. I was becoming very concerned that my car was not going to get the mileage that I had expected. On Easter Sunday I went over to Costco to fill the tank as I was going to travel out of town, but Costco was closed so I went to Fred Meyer and filled my tank and from there I got immediately onto the freeway . In the filling station I reset the mileage counter to zero. On the highway it immediately jumped up to 30 which I had never seen in all the time I had that car. And after I traveled about 10 miles it went to 34. Then I went up into the hills and I never got below 31 or 32 . Now I am just asking the readers what their conclusion is from this.
must be the quality of the gas at Costco. But to make sure, I would buy gas next time at another station and compare.
My conclusion is that the onboard gas mileage readout on your mystery vehicle is wildly inaccurate.
Do both stations use the same percentage of ethanol?
Depending on where you live, you might expect increasing fuel economy this time of year. Fuel economy is worse in cold winter temperatures in general.
Have you been regularly maintaining your tire pressure? If not, then your tire pressure might be higher now due to warmer temperatures, leading to better fuel economy.
As already mentioned, the car’s readout might simply be inaccurate. Use a few tanks at each place and calculate your fuel economy manually to see how those numbers look.
Fred Meyer might have gasoline with little or no ethanol. In addition…they may have their tanks filled with warm weather gasoline which gets better mileage. I’m just echoing what has already been said here.
My conclusion is that the onboard gas mileage readout on your mystery vehicle is wildly inaccurate.
Even though the on board computers have gotten much better than in the past the only way to get MPG is miles traveled divided by gallons used.
+1 to @VDCdriver . The only way to accurately calculate your mpg is to see how many miles you went vs how many gallons you put in at the pump.
My Acura’s mileage estimator is routinely off by 2 or 3 mpg, and sometimes by a lot more. I remember once it told me I was getting 70mpg, which would have been a minor miracle.
I agree. Buy one tank from Fred Meyer and set the trip odometer to zero before you leave the gas station. When you fill up next, got to Costco. Record the mileage on the trip odometer and then set it to zero. Go to watever station you want to next, but record the mileage on the trip odometer again and set it to zero. Calculate your mileage from the two tanks and compare them. Make sure that the driving routes are the same for both tanks, and you have a valid comparison of gas mileage with the two gasolines.
Without knowing your driving habits, it would be a guess on our part. Two people can have mileage vary widely with the same car depending on how they drive.
shadowfax
My Acura’s mileage estimator is routinely off by 2 or 3 mpg, and sometimes by a lot more. I remember once it told me I was getting 70mpg, which would have been a minor miracle.
But would that have been only very briefly, and shortly after filling up? That’s the only time I’ve ever seen goofy numbers. After that my CRV settles down into surprisingly accurate figures.
But would that have been only very briefly, and shortly after filling up? That's the only time I've ever seen goofy numbers. After that my CRV settles down into surprisingly accurate figures.
No, I’d expect something like that. This was at around half a tank right after a uh… Spirited run. Actual mpg at the time was probably in the single digits. 
I have heard this comment too from someone who’s noticed that they don’t get as good mpg from Costco gas as other venders.
I have no idea how this could be though, and have always tended to discount the idea. But you’re now saying the same thing. hmm … well, there’s more to learn about this topic I guess.
Maybe do some more experiments where you drive exactly the same route at exactly the same speeds at exactly the same time of day (so the traffic and ambient temp are the same). Then average all the readings, comparing Costco gas vs another brand. That’s about the best you can do to prove your hypothesis that Costco gas is somehow reducing your car’s mpg.
I’ll provide the opposite experience; I probably fill up at Costco 3/4 of the time or so. Haven’t seen any mileage drop. That said, I do get premium so there may be a difference. I’ll have to see if my MR2 gets different mileage using their regular gas.