Replace worn tires with the original tire size used by Honda. Avoid bigger tires, as Perronne recommends. Pay a mechanic for a wheel alignment because that operation requires computerized heavy equipment.
Make sure your tires are properly inflated, and don't use metal rims or hubcaps because they add weight to the car. Use plastic wheel covers.
Remove all extra weight in the car, including any tool sets, computers and equipment you are keeping in the trunk. Avoid running the air conditioning.
Replace the spark plugs, wires, air filter, fuel filter, oxygen sensor, oil and oil filter as needed. Add an oil additive to improve motor compression for older cars. Use only oil recommended by Honda and maintain the oil at the proper level. Avoid overfilling the oil because that fouls the plugs and reduces mileage.
Start gradually from red lights or stop signs and accelerate slowly but safely, always watching other traffic. Avoid touching the brake while driving unless absolutely necessary. Avoid stop-and-go driving and take the highway when possible because highway driving improves gas mileage.
Drive at 55 mph on the highway as long as it's legal. Maintain the minimum legal speed on the freeway, usually 10 mph under the posted speed limit. Use gravity to your advantage as you drive. Avoid accelerating uphill, let the car accelerate as you move downhill and let your momentum carry you past the base of the hill and partly up the next one.
Use carburetor cleaner to clean the carburetor inside the air filter housing while letting the motor idle. Remove all gunk from your carburetor for a significant increase in mileage. Use fuel injector cleaner in the tank.
Adjust your idle to original levels, and make sure your cooling and charging systems are functioning properly. Ensure that the vacuum level on the emissions control equipment is correct because the vacuum affects fuel flow. Ensure that the fuel pump is working and that the fuel pressure is correct. Maintain transmission fluid at the proper level.Some things about improving gas mileage for your Honda Civic are obvious. Don't race the engine while sitting idle, and don't race up and down the streets at 80 miles per hour. Other ways of getting better fuel economy are not as obvious. Craig Perronne of Off-Road.com writes that tire considerations are important in improving gas mileage, something that isn't really intuitive. If you haven't already purchased but are still deciding which Honda Civic to buy, remember that hybrids are best for fuel economy, manual transmission gas models are second best, and automatic transmission gas models are third.