Stick with brand name fuel. Cheap and off brand gasoline can affect mileage.
"Top Tire" fuel seem to run cleaner, better and give better mileage.
I've had the best with Shell fuel, but that could be a regional thing. Try a different brand for a
tank or two.

My 2¢

 

Great info Dennis.
I like Shell very much but prefer and for the Mazda use Chevron.
Both of these have proprietary formula's that really work as claimed.
For Racing or performance enthusiastic mountain driving I will use 76 Racing or VP, SOC or S&L . My engine for best power is calibrated with a custom ECU calibration for 100octane and for best overal 93-95 octance adjusted.

 

I consistently drive the same roads and I get on average 30-32mpg with regular 87.
I purchase fuel at stations with the best price but it's generally Irving (Canadian) up here in Maine.
It's cheaper hauling it down from Canada than bringing it up from the Gulf or NJ.
My mileage doesn't change much at all even after oil changes, new tires or air filters or the colder winter weather.
I do increase to 34-35mpg range when on the highway, despite the increased speed I drive...go figure.
I got 36mpg once when headed up north on the Interstate which is fairly flat, and I was averaging 80mph.
But then again, I got 44 mpg on the same road with my SV1000S averaging 135mph

 

Gasoline is gasoline. It all comes out of the same cracker towers. The difference is the blending that is done afterward. There is a variety of octanes, cleaners and other chemicals added. Then you have ethanol. Winter blend fuel has more ethanol unless you get ethanol free fuel. Ethanol has less BTU so it's less powerful. Your mileage may have dropped merely becase of seasonal blending. I watch my mileage as unusual drops may mean a mechanical issue building.

 

The standard gasoline expansion is 3.4% per 50 degrees F change in temperature. That’s the temperature of the fuel in the underground tank, not the ambient air temperature. So comparing fill ups in winter versus summer would probably mean an average change of less than one mpg. Not the OP’s problem here, but it could combine with other factors to drop his mileage more.

 

Yes good information. But the newer plastic fuel tanks and the current F/I system specifically on a MAZDA as this is what this forum and thread is about the temperature changes from winter to summer in months driving environments in the US with the larger population of roads and highways tarmac surface would not equate to the normal reduction of MPG based on fuel tank temperature. Racer at least Professionals account for this as well specific gravity when calculating any needed ECU or fueling changes. This information also applies to daily drivers.
I may also add as support so there is not misunderstanding the top 4 Mazda forums when the topic of MPG comes up members post all over the world and at different season and they all sem to be pretty much the same.

But I still again comment you on your internet found information! Or you must be a Pilot?