Gas mileage decreasing

I used to get 30mpg, but now it’s only 24-25mpg. I drive a 1994 Toyota Corolla with ~69,000 miles on it and drive only about 3000 miles per year, mostly around town. My gas mileage used to go down every time the dealer performed milestone maintenance, but then it would increase again. Now it is stuck at the lowest mpg ever. No engine light and no leaks. The dealer responds to my questioning this with a shrug. Any suggestions?

The arrival of colder weather always reduces gas mileage a bit. If you’re using gas with 10% ethanol it will decrease your mileage by about 10%. If your tires are under-inflated (another thing that goes with cooler temperatures) they, too, will contribute to reduced gas mileage.

When was the last time the spark plugs were replaced?

How old is the thermostat? A thermostat that’s stuck partially open will cause a loss in fuel mileage. Also, a defective coolant temperature sensor for the computer can cause a loss in fuel mileage.


I second the thermostat diagnosis.
Only 3000 miles per year, but how many drives per week?
Every time you warm up the engine the thermostat has to cycle at least once.
Most daily driven cars should have the thermostat changed every 4-5 years, IMHO.

Thanks for your input. I live in SW Arizona and we don’t have much cold–the coldest it’s been in my garage since last Jan./Feb. is probably 60-65? I don’t drive all that often, sometimes once a week, sometimes more, but usually no more than 20 miles. I think all gas here has 10% ethanol and has for a long time, so I’m not sure that can be the problem. My tires are inflated properly; that’s the first thing I checked. Last major work was 2/27/08, about 6,000 miles ago. At that time brake and transmission fluids were flushed and replaced, spark plugs, air filter, timing belt and valve cover gasket all were replaced. As with every other time that the dealer fooled with it, the gas mileage went down by about 5 mpg, but this time, instead of coming back up with use, it is decreasing. Coolant flush 10/07. In 4/04 there was an injection flush (Part replaced-Carbon Cl) and the fuel filter was replaced. 1/02 replaced radiator hoses and drive belts. No record of a thermostat replacement.

I made an appointment with an independent garage to get it checked out with his computer, but in the meantime he suggested that I add Techron to the gas tank. Problem is, the directions are to add it to a nearly empty tank and I had just filled up when he suggested the Techron and haven’t used that much gas yet. He also suggested that I take it for a longish drive on the highway, but thought I would save that for just after adding the Techron. Does any of this make sense? I’d like to save the $ for the computer check, if possible. Should I add the Techron to a nearly full tank? I bought the 20 gallon treatment even though my tank holds 13 gallons, planning to add the whole bottle anyway. Might that be a problem?

As for the thermostat, if there were a problem with it, wouldn’t it show up in the temperature gauge? This car has a simple dial gauge, and it runs in the mid-range after warming up.
Again, thank you for your help.

It is unlikely that just a “computer check” will find your mileage issue.

Good to know. He proposed the computer check because there are no other symptoms. What do you think of the Techron idea?

Be certain that your tires are properly inflated. The information is on the left door pillar as to the correct tire pressure. Spend $10 and buy a good tire gauge. I would bet that since you are only driving 3000 miles a year around town, the tires aren’t checked that often for proper inflation.

A stuck thermostat will not necessarily show up the temperature gauge.

The engine managagement will not go into the closed loop mode until coolant temperature reaches 170 degrees or higher. If a stuck thermostat only allows the coolant temperature to only get up to 160 degrees the engine management stays in the open loop mode. So engine runs rich because the computer thinks the engine is cold.

People get fooled by this because they figure as long as the temp gauge shows a reading and there’s heat coming out of the vent system that everything is fine. When 160 degree coolant passing through the heater core is more than enough to heat the passenger cabin.


I go with the shrug. If you drive 10k a year, you are out $200 + dollars from where you would like to be. Trying to satisfy your expectations will blow that on your first shop visit. Lots and lots of stuff brings mileage down and what you’re talking about is not worth chasing - especially for a 1994. Save your money for what’s coming. A 94 is ready to start the parts replacement cycle.