2000 Ford Focus mileage getting worse
I know my car is not very good with gas by design, but it's getting worse lately. Over the summer I managed to hit up to 483km (around 28MPG US) with 30/70 highway/city. Then when Fall season hit I suddenly dropped to just barely above 400km (about 24.5MPG US) with the same city highway ratio. I know the cold weather hit my mileage in a major way but that is still pretty good considering I drive more in the city rush hour. I always use Shell gas but now my mother borrowed my car for a week and she filled it up with gas from Petro-Canada. Now I just got home from work and I already used up 1/4 tank (it's just a 40km round trip)! So clearly, the mileage is much worse in this latest tank. It's really annoying that this car sucks gas like it's nothing ...
In terms of driving, I drive smoothly (rpm below 2000rpm as much as possible), just had an oil change a few weeks ago, air filter is still clean, tire pressure is in spec. I dont know anymore, since September hit my mileage has been getting worse and worse ... now this Petro-Canada gas might be a factor too and it got cold here in Canada quite fast in September and the gas stations might have switched to their "winter" gas. Anyone else having this? Are there anything else I can do to improve this abysmal fuel economy?