• Added a cold air intake system and low-restriction air filter
• Installed 33-inch-diameter tires
• Replaced the rear end gears so the engine speed is the same as it was with the factory wheels and tires.
Considering all of these changes, my fuel economy has actually dropped from 16.6 MPG on the highway to just 14.8 MPG. What do you think could be the cause of my drop in fuel economy?”
Halderman: Where should I start? First, when changing parts with aftermarket parts, remember that there was a good reason for the way the original parts were designed.
For example, consider the first two items in your list. I have seen where a low restriction-type air filter and exhaust have improved fuel economy, but they also increase noise and may not be able to work with the engine management system.
If changing something, try to make only one change at a time. This way if one part results in lower fuel economy, then you would know right away. When you change more than one item, then it is a guessing game as to what really caused what.
One of the reasons for the reduced fuel economy could be that the engine computer cannot make a large enough change to the air-fuel mixture due to the extra airflow through the engine.
I also noticed that you live in a high altitude area where the air is less dense. One way to determine what is going on here is to use a factory-level scan tool to check the oxygen sensor(s) and other factors, such as spark knock information.
If the engine was operating too lean or pinging (spark knock), the computer will reduce the spark advance. This could be the cause.
The last two items on your list I think were done for looks and not fuel economy.
Installing larger wheels and tires almost always results in lower fuel economy due to the increase rotational inertia. This means the engine has to work harder to rotate those larger tires.
I have a saying that comes to mind that says “keep it stock.” This means that the vehicle manufacturer does everything it can to improve the fuel economy of its vehicles. Usually whatever we do often results in lower fuel economy.
Wheels: What should Bill do?
Halderman: I think the large wheels and tires are a major reason for the drop in fuel economy.
However, to go back to the way it was will be expensive. I think he should see a professional technician and ask that the computer system and sensors be checked for possible causes.
Maybe something happened when these parts were installed. I think the air filter and the exhaust system themselves are not a cause of his reduced fuel economy — especially since he lives at a high altitude where the extra airflow would tend to be more beneficial.
James D. Halderman is an ASE-certified master technician, a member of the Society of Automotive Engineers and the author of 12 textbooks. Submit your automotive care questions to Halderman via e-mail at jhalderman2@juno.com, or write him in care of: Wheels, Marketing Publications Department, Dayton Daily News, 1611 S. Main St., Dayton OH 45409.
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