Big change in gas mileage for a durango

My 99 dodge durango used to get 22 mpg on the highway, and 17 mpg in the city. Now it gets 10 mpg in the city, and maybe 16 on the highway. So what’s up with that? I had an oil, lube, and filter back in July 2010. It’s got a new batter. what gives?

Probably one of the following:

Bad thermostat (stuck open)
Bad coolant temp sensor
"Lazy" oxygen sensor(s) that are under-performing but not bad enough to set “check engine” light.

Any one of these can result in engine running too rich and drastically reducing mileage.

When you say “used to get . . .” what does that mean? How long ago did it get 22/17?

Was it last month, last year, or in 1999?

Along with the things jesmed suggested, I’d wonder about the age of the spark plugs, the air pressure in the tires, the mileage on the truck, and whether or not the gas you’re buying contains ethanol, and how much stuff you carry around with you inside the truck.

Lots of things can affect fuel mileage.

You are getting about the same mileage as the vehicle is supposed to get. A quick glance at the EPA’s fuel economy ratings tells us that the 1999 Durango is rated to get from 11-13 MPG city and 15-17 MPG on the highway. Basically you’re current mileage is almost dead on for what it’s supposed to be getting.

No offense to the OP, because he/she is just one of many. But there seems to be a reoccurring theme on this site where someone will say “I have make/model that used to get unusually high/borderline unrealistic MPG numbers, and now it’s really low” But when you check the revised EPA ratings for that make/model, almost without fail the new lower mileage that the OP mentions is usually right in step the with EPA estimates. Just something I’ve noticed.

It depends on which engine the OP has. He may have the V6, not the V8…although I see now that the V6 EPA rating isn’t much better than the V8…

Driver has a lot to do with mpg figures, so if same driver is noticing reduced mpg on same route, then it is something to sleuth out. In our house, one particular driver always gets much better than EPA estimates ( and those EPA figures are just estimates).

Any recent repair work besides the routine maintenance mentioned?

This change could also be caused by the change in season. Depending on where you live, if you get a real summer and a real winter, you will notice worse fuel economy in the wintertime due to the lower temperatures and what they do to a car.

Is this vehicle new to you, or have you had it a long time and it truly has never done this badly? Have you had it during previous winters? Cold temperatures can have a profound effect on observed gas mileage. Warming it up in the morning will have a profound effect on observed gas mileage. By the way, it’s good that you have treated your truck to an oil change six months ago (it’s probably due again by now, and I hope you monitor the oil level between oil changes. That’s a huge cause of discussion on this forum), as well as a new battery, but neither will normally have an effect on gas mileage.

It’s getting what it should get. You now do a better job of dividing. You used to do it by hand, now you use a calculator.