You always get better gas mileage in the spring and summer unless you're using your air conditioner which uses more gasoline so your gas mileage goes down when the cars air conditioner is on.
But when the air conditioner is off and the weather is warmer your engine doesn't need to use as much fuel to power the engine.
During cold weather your engine needs more fuel to keep that engine running so you'll need to fill up your gas tank more often.
I notice my gas gauge always drops more in the winter than it does in the spring and summer.
Also when it's really cold out and you first start your engine the engine idles high and uses more fuel because it needs to run richer for at least a few minutes until the engine gets to operating temperature.
But in the spring and summer the engine warms up faster and doesn't need as much fuel to start it and keep it running.