2002 Toyota Corolla - MPG drop

My 2002 Corolla LE 150K miles has always been well maintained. Recently noticed gas mileage dropping a bit from 27/38 mpg to 23 city driving. So replaced spark plugs (NGK iridium), (Denso) upstream & downstream oxygen sensors. Air filter is clean. No codes are on. But mpg didn’t improve. Any suggestions? Will new Denso Mass Air Flow sensor help?

Ho closely have you monitored the tire pressure and did you check for a frozen caliper?

You replaced all those parts? Even when there was no Check Engine light?

That’s called shot gunning parts at vehicle.

One thing that can cause a sudden drop in a fuel mileage is a stuck open/broken thermostat.



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While you are throwing parts at it try a pcv valve

Exactly when was it 27/28? Has the weather changed? Was your mileage carefully measured both at 27 and at 23? It is not easy to get a precision measurement of mileage under identical circumstances.


+1 to the thermostat.
I change that and radiator cap every 10 years as preventive maintenance.
Use only OEM, many aftermarket units are junk.

If the tires are inflated to the proper pressure and the thermostat is allowing the engine to get up to operating temperature, then there may be other factors beyond your control that caused the mileage drop. If the weather is really hot, the air conditioning may be putting an extra load on the engine.
I had the opposite situation a couple of weeks ago. We took a trip.out west from Indiana to the western part of South Dakota and North Dakota. The road mileage in our 2017 Toyota Sienna usually runs 26 to 28 mpg on trips. Through the Dakotas, we averaged between 30 and 32 mpg. I drove the speed limit. When we came back through Minnesota and Wisconsin, the mpg dropped to 27-29 mpg. I used the ordinary 87 octane gasoline that is 10% ethanol and filled the tank at whatever station was convenient. I wondered if the gasoline is blended differently in the Dakotas. However, I don’t think I will move to one of the Dakotas to get better mileage, although I find the Bad Lands breathtaking.
As long as my vehicle is running properly, I really don’t worry about fluctuations in gas mileage.

I have noticed a decline in the fuel economy of my personal vehicles, as well as my work truck. I attribute this to higher levels of ethanol in the fuel and lower density of the gasoline while refueling due to the summer heat.

If you’ve hit a few potholes lately your car could need an alignment. I saw an increase of about 7% after having new shocks and struts installed, and folks here attributed it to a proper alignment.

One other thought: Did you recently put different tires on the car? If so, were they the same size?
I once had an AMC Javelin. The specifications called for 6.95 x 14. I didn’t have much money, but I got a bargain on four barely used 7.75 X 14 tires. They fit, but they threw the odometer and speedometer off and I had to factor that in when I calculated the gas mileage.