Subaru 1995 has greatly decreased gas milage! WHY

My 1995 Legacy has suddenly gone from 220 miles a tank to less than 160 miles a tank!

I’ve had new correct size tires, new starter and tune up recently. What can cause this problem?

Have temperatures in your area begun to drop? That could explain the drop in mpg to some extent.

Have you verified that those new tires have the correct air pressure in them?

Or, another likely cause is a cooling system thermostat that is stuck in the open position, thus causing the engine to run on a much “richer” fuel/air mixture. Have your thermostat checked/replaced.

Were the old tires’ size incorrect? How far off?

We prefer to measure miles per gallon, not miles per tank. Your Legacy should be able to go MUCH further than 220 miles per tank, much less 160. The fuel gauge is not accurate enough to use it as a measurement of fuel mileage.

The fuel gauge on my '96 Legacy reads empty when there’s still more than 1/4 tank of gas. If I went by the gauge, I wouldn’t go much more than 250 miles on a tank, but I know I can go 350 or more without running empty.

There are many possible reasons for the decrease in mileage.

What did the recent “tune up” include? What kind of tires did you buy?

My guess is the reason for the mileage drop is either in the tune-up or the tires, or a combination of the two.

Okay, temperatures have dropped but this started before the cooler nights began. I don’t know the size of old tires, they were Kumho. New tires are Kenetica Kenda Radial.
Drop in mileage started before both tune up & tires. The mechanic said the spark plugs were really worn down so the fuel mix was rich. Tune up included new plugs, air filter, fuel filter.
I haven’t checked the air pressure in the tire, I can do that today.
I have never gotten over 300 miles to a tank. Usually it seemed around 24 miles to a gallon.

Thanks for all the help!

Um…are you running this tank to empty to track the mileage? Allow me to suggest this is a very bad practice. You may want to check you fuel line pressure.

There may be a faulty coolant temperature sensor causing this to happpen. This is a different sensor than the one for the dash gauge and has two wires going to it. It can make the ECU think that the engine is always cold and run way too rich.

Thanks for the suggestions! I will have the cooling system thermostat checked, as this has been mentioned twice here. I gauge the MPG by watching the miles on the odometer.
Thanks again- so glad to have a place to ask questions!

You need to check fuel mileage the right way. Record the mileage and gallons at each fillup. Do the math. Compare mpg, not miles per tank.

“Kenitica Kenda Radial.” OK. Never heard of that brand before.

I have to assume you purchased the least expensive brand you could find. Sometimes that’s a good thing, and sometimes it’s not.

Without more information to go on, it’s really tough to say.

The tires were supposed to be mid level.