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Dramatic drop in gas mileage per gallon

My 2003 Nissan Sentra used to get 35-38 mpg in freeway driving, and then rather suddenly it was only getting 25-27 mpg highway. Changed spark plugs and anything else mechanic could think of. Better for a few days and then back. I’d get 100 mile to quarter tank, now 100 to 1/3 to 1/2 tank.

On Car Talk I think I heard (ll-15-2008) that it might be the thermostat and/or something else but was driving and couldn’t write it down. Is that true and what is the something else that it could be? Are these expensive changes?

Some more info is needed.
It sounds like you’re basing fuel mileage on the dashboard gauge. This is not a reliable way of doing this at all. You need to use the fill and tripmeter method.

Any Check Engine Light on? If not, you may not have a problem because if your mileage was suffering that bad the CEL should be illuminated.
Has anyone pulled the codes? Again, if mileage is that bad something should be present.

Guessing at a problem can get pretty expensive.

In addition to OK’s good questions and comments I will also suggest that it has recently gotten cold in many part of the country and that means different fuel and driving conditions that both mean lower mileage.

To add to Tester’s measurement comments, the only way of getting accurate measure of mileage is by checking the miles since the last fill up and the gallons added. Even then you should be suspicious until you have done it a few times and had consistent results.

A lot of things can cause a drop in mileage, the thermostat being one of them. If you have a bad thermostat, you will notice that you don’t have a very good heater and the temp gauge doesn’t rise to the level it used to.

You could have a fuel leak, a slipping transmission/clutch, sticking brake calipers, bad wheel bearings, low tire pressure, new stickier tires

It was better for a few days after tune-uppy things were done; so, it must be something beyond that.
Have your mechanic examine some of the spark plugs. If they are sooty, or black, the engine has been running rich. What could make an engine run rich? 1. Thermostat stuck open. 2. Fuel pressure too high (fuel pressure regulator). 3. Engine coolant temperature sensor reading “colder” than actual. 4. Other stuff.