Why did my gas mileage suddenly drop so much?
Removing a bad catalytic converter and switching to lower-quality fuel can cause a dramatic decrease in gas mileage. This leaves drivers getting poor fuel economy on 4-cylinder engines and struggling to figure out whether the drop is a normal sign of age or a mechanical issue requiring repair.
What the numbers show
Counted from the threads quoted below. How we count
- This comes up 17.4× more often in r/AskMechanics than across the site as a whole.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
Found another way around this?
Share your steps and trade-offs so fellow engineers don't have to reinvent the wheel. No account required.
- 1keeping up with tire pressureDid this help you?
- 2using an O2 spacer on the downstream sensorDid this help you?
- 3adjusting timing by loosening and turning the distributorDid this help you?
- 4replacing worn spark plugs, fuel filter, distributor cap, and rotorDid this help you?
- 5testing various brands on a consistent daily routeDid this help you?
- 6switching to alternative gas station brands like Shell, 76, or ChevronDid this help you?
- 7buying a bottle of fuel system cleaner or solvent to run occasionallyDid this help you?
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“for my past couple of fill ups I have been getting 12-13mpg, granted it was about half city/highway driving but I feel like 13 mpg is pretty bad for it to be a 4 cylinder.”source ↗
“I’ve recently had the catalytic converter removed and everything welded up due to having a bad cat.”source ↗
“Is what I’m experiencing completely normal for a 25 year old car. Or is this maybe a sign to a bigger problem? And how do I go about fixing it”source ↗
“i swear every time i fill up at arco the tank seems to disappear faster than when i use 76 or shell.”source ↗
“There is a difference in mpg with some being terrible.”source ↗
“arco always feels like it goes quicker.”source ↗
“I have been getting very bad gas mileage and I don’t have a check engine light on . Is there anyone out here that can give me some advice what can be?”source ↗
Where this came up
People with this problem also raised
- 2Why is my city gas mileage so terribly low?
- 2Engine RPM drops right after starting unless I give it gas
- 23Why does my car engine cut out after driving for a while?
- 24Why does my car lose power when I step on the gas?
- 3Can I drive with low transmission fluid until I fix the leak tomorrow?
- 2Why did my ad impressions suddenly drop to zero?