What could cause a Honda civic 2000 to waste too much gas?

I already fixed and replaced the following:
Spark plugs
Oil change
Wheel alignment / chamber fix (what is this?)
New tires
This is a hatchback and the car is very fast in automatic. How could I figure this out?

It could be an oxygen sensor. I used to have a Ford Escort that averaged 28-30 mpg. One of the oxygen sensors went bad, and the gas mileage dropped to 17 mpg. Have a mechanic scan your car's computer to see what the problem might be.

Sounds like it's your driving style that uses too much gasoline. Let up on the throttle, lead foot

What is your current miles per gallon figure.

Cold winter weather. My honda 10 l per 100 km in the summer and above 13liters per 100 km winter time.

Wheel alignment might cause some gas mileage reduction, but not a lot. Search toe, camber, and castor for more info.
Start with easiest things first. 1. Document your actual mileage in a notebook. Fill tank, don't overfill it, just normal. Use same brand name station. Write down miles, drive to half tank, fill up, note gallons used.
End odometer - start odometer =miles driven/gallons = mpg.
2.Change air filter and fuel filter if is inline and easy to change, ie not in tank.
3. Carefully inspect airbox and air intake hose for leaks, may need to remove air intake hose to inspect. Inspect MAF connection. Inspect vacuum hoses. If old and worn, just replace them. Do ONE at a time so you don't get them mixed up.
4. Clean MAF with MAF sensor cleaner. Follow instructions, let it fully dry.
5. Change spark plug wires. Get exhaust inspected for leaks.
6. Inspect seal gaskets for gas tank and oil filler caps.
Inspect/test PCV system. Check value might be bad or system clogged. Search on how to test for that car.

With a good handheld scanner you can track the O2 senor readings and may be able to see if outside of tolerances, but I think would be throwing a check engine light if was bad.

Or maybe best to let a shop that specializes in Honda, they may have diagnostic program that can say exactly what is wrong.

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