Winter gas is different also (depending on where you are). Gas mileage will almost always be better in the summer. I had to check on Webmath.com to get your conversions right.

According to them, 400 kilometers (km) Î 0.621 = 248.4 miles (mi). So you used to get 250 miles a tank? That's actually not that great, either. I usually get just short of 300 miles. And 300 kilometers (km) Î 0.621 = 186.3 miles (mi). If you're not even getting 200 miles a tank, there's a problem.

Make sure you don't have any exaust leaks.
Check and make sure your cat isn't clogged up.
Check / replace your air/fuel/oil filters, a good tune up would be in order.
Inspect and clean your throttle body for gunk and buildup.
Are you throwing any engine codes?