Sudden MPG drop, no change in driving habits or gas

My 2008 Honda Fit has been getting 32-34 mpg city and 37-38 mpg highway, with the AC running. Suddenly I have two tanks full that clocked in at 24 an 27 mpg, for mostly highwat driving. I have not changed type of fuel. I have owned the car since April 2011 and had consistant mpg driving everywhere from NY to FLA.

What should I have the mechanic check out first? I have a full warranty, but want some guidance before I just take it in and say “Fix it.” I just turned 63000 miles.

You may need one more tank to be sure this isn’t an issue of the pump shutting off at a different time. However, let’s assume you’re correct.

Is your tire pressure low? After a long drive, is one wheel much hotter than the others (in which case that brake is sticking on)? Do you have a check-engine light on? Are you up-to-date on all of your maintenance? If this is an automatic, is your car properly shifting into the highest gear, with the torque converter locking up? Are you using the proper grade of oil?

If the CEL is on have the codes checked and it will probably lead you to your problem. Such a drastic drop in mileage is often associated with a bad 02 sensor. I’ve had them go bad and cause a 25-35% drop in fuel efficiency.

Did you have anything done to the car just before the mileage dropped? A drop like this has to have an assignable cause, and it often traces back to a maintenance that was just performed.

The first thing to have checked is a stuck open thermostat. If the engine doesn’t come up to full operating temperature the computer will believe the engine is still cold and stay in the open loop mode.

If the thermostat checks out okay, then the next thing to check is a defective coolant temp sensor for the computer. If the sensor has failed where it’s telling the computer the coolant never gets to operating temperature when it actually does, the computer will believe the engine is still cold and stay in the open loop mode.

In both of these cases the engine will use more fuel.


How are you determining your mpg?

Check it a few more tanks of gas. It could just be a fluke. If the problem continues, the first thing is to bring all your maintenance up to Honda specs for your car. Look at your owner’s manual for the specs.

And think of what has changed recently. Have you driven in extra-dusty areas lately? A clogged air filter for example could cause this.

After that, ask your mechanic to show you any stored codes.

If you are using the Honda computer to determine gas mileage, don’t depend on it. Reset the trip odometer to zero and record the mileage when you fill up next. Using that and the gas volume on your receipt, you can determine the gas mileage accurately. I don’t know how you determined gas mileage and thought I’d mention this if you do it the other way.

One low tire can kill fuel mileage so tire pressure can be a consideration.

You should be getting about 29-31 MPG overall with this car. If it was just one 27 MPG tank, then I would chalk it up to an anomaly, and it would still in the ballpark. But 24 MPG is a bit low. As the others have mentioned the methodology used in determining fuel mileage may be a factor here. Has anyone else been driving the car?

I would also try another gas station, or different gas. It possible your station just switched suppliers who are using a bigger blend of ethanol.

Do you have a locking gas cap?

london, I’m pretty sure all Hondas have a latching gas cap flap that can only be opened by pulling a release lever by the driver’s seat.

I have a full warranty, but want some guidance before I just take it in and say “Fix it.”

I don't know what you do for a living, but let's say you lay concrete and have been doing it professionally for the last 10 years. <b> Would you want the home owner telling you how to do the job? </b>  

Tell the mechanic what you have noticed and why you brought your car in.  Let the pro do his job.

@ jesmed
No, sorry I have a Fit and neither the cap not the latch is lockable.

Strange, all Hondas I’ve owned have had latching flaps. I guess the newer ones don’t.

Anyway, I agree with Tester that it’s most likely a stuck open thermostat or a bad coolant temp sensor. Either one will severely impact fuel economy.

Okay, I have tried 3 more tanks of various gas brands. Highway MPG now varies from 24 to 25. I am not using an onboard computer to do MPG. I reset trip odometer each time I fill up and make my own computations. I do not top up the tank, but there is not enough room to make that much difference, and I have never topped off tank, anyway. No one else drives the car and it has not been worked on at all since the drop started, except to have oil and filter changed AFTER the drop occurred. Now I have had 6 fills of different brands, and the mileage is still way low. Tire pressure has been checked and is not only correct, but also at the same level as when I was getting much higher MPG. There is no CEL on; I would have long since gone to the shop if there had been, thank you! Air filter is fine, been checked. No dusty area driving.

I am inclined to have the thermostat and coolant temp sensor checked as Tester suggested. Many thanks for all the good ideas.

Have you had your transmission fluid changed yet? This could be a transmission issue instead of an engine issue. BTW, there is a code if the ECT (engine coolant temp) does not come up withing a certain time frame. You could also have a fuel leak.

My 2010 FIT dropped 20% immediately upon moving to a much colder climate. I checked tire pressure and changed air filter, changed gas stations. Still off by 20% ( 10 miles per gallon). Is it just the cold environment? Car is parked in a garage when not driven

You should start your own thread.