My car is 8-10 in normal usage, maybe 12+ for long trips.

Your car should do 8-10 km/l in traffic... or more, and 12-14 km/l on long trips.

No, gas savers are a big waste of money and time... absolutely all of them. Spend your money on oil changes, tune-ups and clean filters (air, oil, fuel) and proper maintenance.

There are some items (K&N cone filters, Michelin low-rolling resistance tires) that DO save gas as a SIDE-EFFECT, but the amount of fuel you save STILL won't cover the cost of upgrading to those items.