Andrew Tsai

Hi, I have had a rsx for about 8 months now, and when i bought it it came with a cold air intake and a loud exhaust and 18 inch rims, but I have never gotten 35 mpg like I am supposed to on the freeway, and in the city, well, the suburbs, I only get about 19 miles per gallon... Is there something wrong with my car and how can i get my car to get better mpg, at least what Acura advertises.

knavekid

Something is wrong. I get 25 city with my Type-S and I drive fairly aggressively.

It could be your car has a bad or missing sensor and is running in safe mode, which can default to a rich air/fuel mixture. If your check-engine light doesn't turn on when you start the car, it may have been removed. You might take it to Autozone for a free CEL code scan and they can tell you if there are any error codes. Lastly, take it to a good import mechanic or the dealer for a complete analysis and tune.

ParaSurfer1979

If he has no check engine light in his dash then there'd be no codes to check for. How many miles does the car have? It may be time to swap out the spark plugs. This would help drastically. What brand gas and octane are you running? You may also want to use Sea Foam treatment which helps a lot and is a simple process that you can see many owners of RSX-S' use it on youtube. All these things will bring your mileage back to what you're used to see'g.

MeanBlackCLMachine

The computer can still store codes and not show it in the form of a CEL

knavekid

The check engine light should always be on while starting the car. If it is not, it has either failed or been removed. This could be the case if the previous owner wanted to hide problems with the car. If the CEL has been disabled, the only way to find out if there is a CEL is to hook up an OBD2 scanner.

C25A1guy

More information is needed to properly find the issue. For sure a couple of things that might hurt your mileage. Those 18" rims probably are heavy, cheap rims. Heavier wheels will require more energy to start moving. That cold air intake filter may be dirty. The exhaust is probably zapping some of the low end torque that the autos have to live by. Until we get more info (miles on car, type of intake, filters clean, etc.), I would try a few things first to help:
1: Tire pressures. Proper tire inflation is critical to good fuel mileage. In your case, you have 18" rims and tires which will probably be stickier performance tires (more rolling resistance) which doesn't help the matter much. In any of my Hondas, they never leave to house without less then 35 PSI cold.
2: Clean air filter. Considering that you have a cold air intake, depending on the brand of intake, you might be able to clean and reuse the filter. Most, if not just about all name brand intake kits have reusable filters. Check for what kind of intake kit it is and report back
3: If the car is over 105k miles, change the spark plugs. Stick with the original NGK Laser Iridium plugs.
4: Check your fluids. Dirty ATF will cause decreased fuel mileage. If the fluid is more brown then it is red, change the ATF. Make sure you use only genuine Honda ATF. You may want to do a 3x drain and fill with the new Honda ATF-DW1. Dirty oil or using incorrect oil weight will also cause decreased fuel mileage. The engine calls for 5w20 IIRC. You may use 5w30 without issue. The problem is from many people with old ways of thinking on higher mileage cars running super thick oils like a 20w50. I run 15w40 in my RSX, but that's because when it's running, its staying around 6.5-9k rpm at all times for 30-45min so I need the thicker oil for the protection. Your car is an automatic street car. You won't need that thick of an oil.
5: Switch out the exhaust for an OEM or a "quieter" exhaust system to bring the torque and powerband lower in the RPM range. I would go deeper into this, but it would require you to have a basic understanding of basic mechanical engineering and fluid dynamics.