Very Low gas mileage

My daughter lives 2000 miles away so I am trying to diagnose her very low gas mileage remotely. She has a 2010 Hyundai Accent, bought new, with about 36k miles on it. About 6 months ago she noticed very low gas mileage. It should get about 30+ MPG but she's getting about 22 now. Since then she has had it checked by 2 dealers and a local mechanic and no one can tell her what's wrong. She recently changed the tires and the air filter and has seen no improvement. The check engine light is not on. I'm think of changing or at least checking the spark plugs when I visit in October. Any suggestions would be greatly appreciated.

Did her driving pattern change at all? My own mileage drops considerably from summer highway to winter stop & go use.

In addition to what Docnick stated, another part of the equation could be the rolling resistance of her tires. Many–perhaps most–people never consider rolling resistance when buying tires, or perhaps they balk at buying the ones with low rolling resistance because they tend to be more pricey.

Possible causes: thermostat stuck open, or faulty coolant temp sensor. Thermostat stuck open will cause engine to run cold, at lower fuel efficiency. Faulty coolant temp sensor will fool computer into thinking engine is running cold and sending more fuel through the injectors than necessary, also reducing fuel efficiency.

One or more brake calipers might not be releasing fully causing drag. A low cost infrared temperature tool can help diagnose. Simply point the red dot on an area and you get a temperature reading. If one wheel, or disk brake rotor, is significantly hotter than the others you have found the offending wheel. You can get a less precise reading by simply touching the wheel with a finger.

The first step is to determine how she is determining what her mpg is. If she is not looking at the trip odometer and dividing it by the number of gallons she puts in the car, she is not getting an accurate mileage result.

I’m eliminating mechanical problems because the dealer has looked the car over. I assume (possibly incorrectly) that the car is in good mechanical condition because of this.

Did she EVER get 30 mpg? Or is she just checking it now for the first time? If she drives mostly in the city with stop and go driving she could very well see 22 mpg.

Not to insult your daughter’s driving but the person driving the car has the greatest effect on the mileage the car will get. If she is not smooth and steady applying the gas, accelerates quickly, stops short, can’t hold a steady speed in traffic she will get lower mpg’s in any car. Make an honest assessment of her driving style and where she drives - city only, city/highway or combined and make your judgement from that.

Also, has she changed brands of gasoline recently? Or perhaps she SHOULD try another brand.

Is she loaning the car out to another driver ? . Driver habits have a huge influence on mileage.

I seem to be in the agreeing w @jesmed1 mode today … lol … spot on, those would be my first suspects too, the thermostat or the coolant temp sensor. CTS’s are fairly rare to fail, so focus on the thermostat. Maybe it just needs to be replaced. On most cars that’s a modestly priced repair.

Once the thermostat function is known good, place a little mark on the dash board coolant temp gauge when the car is at full operating temperature. Then if it ever deviates from that mark you’ll know the thermostat needs replacement again. Thermostats, moving parts with seals and being constantly immersed in coolant, are not a life-of-the-car item, and usually need periodic replacement.

Agree with the others but I’ve had a lazy o2 sensor really cause bad mileage. It wouldn’t necessarily set off a mil light though so they’d need to hook the computer up and see what the sensor is doing. You would have thought they would have already done that.

Bing is right, have a mechanic put a scan tool on it and see if the car’s fuel injection system is going into “closed loop” mode as it should after it warms up. If it’s staying in “open loop” mode it will be using too much fuel. Once the mechanic finds out it’s staying in “open loop,” then he can figure out why (bad sensors, etc).