Recent terrible fuel economy in my 2011 250 AWD
 

Hi all - new to the forum and was hoping for some insight. I have a 2011 IS 250 AWD (36,000 miles) that seemingly runs fine (no engine noise, check engine light NOT on) but whose MPG has plummeted in the last 4 weeks or so. Per tank average MPG has dropped to 12.5 MPG from about 21. I didn't notice this initially because I drive so little. The average miles per tank is listed on the display is like 207.


Does anyone know what could be going on here? It will likely be going in the shop very soon. Thanks in advance.

It depends how much city driving you are doing in relation to highway driving and a multitude of factors (i.e. driving style, fluid conditions, tire conditions). I would start by checking air in the tires and pulling the negative terminal off the battery for 30 mins and let the car's ECU relearn your driving patterns once you reconnect the terminal to the battery and resetting your windows. I would also recommend checking/cleaning/replacing the air filter and maybe clean the MAF sensor and drive around and see what kind of MPG you're getting at each fill-up.

I recommend doing a more mathematical way then looking at the dashboard display values. A simple way to calculate this is to reset the trip on the odometer and fill the car up to a full tank, then run the tank down to where you usually go to fill up. At the station after filling up the car, whip out that cellphone and take the number of miles logged on the trip and divide that number by the gallons that you filled the car up with (on the pump or receipt). That number will tell you the exact miles per gallon that you drove.

I've learned that the display can have a bit of deviance to what they read out in terms of MPG. My display on my 2007 IS 250 AWD will show 24 MPG after a week of highway driving with light amount of city driving and the actual calculation would be 22 MPG. So the display has a roughly +2 MPG deviance, while the Range Approx. (depending on driving conditions) is roughly spot on.

If you're driving only city and a lot of idling, your mileage is gonna plummet.
The computer is simply calculating range based on your mileage, and it's also simply calculating mileage based on how far you've driven for each unit of gas (be it gallon or liter or even milliliter) .
If you're simply stopped at a red light forever, your MPG will drop to 0 because you're driving 0 miles while the engine is still sipping a little bit of gas to keep running, so 0 over whatever amount of gas will be 0.

According to the shop...
 

The vehicle's thermostat is toast. The car was running at about 140 degrees instead of closer to about 190. Apparently fuel was being wasted (raw fuel in the exhaust system) as a result. After a new part(s) labor, and a $75 "diagnostic fee", the price will be $275 plus tax. Seems high to me.

What shop? don't you have warranty ?
did you take car to dealer ?
have you ever changed your plugs ? air filter?
whats your driving habit like ?
like others have said, there's alot of factors that could affect gas mileage, but thermostat ?
let us know how it goes

Why the incredulity? A stuck-open thermostat will certainly cause a big drop in fuel economy, especially in cold weather.

The only surprising thing is that the original poster didn't say anything about poor heater performance, or notice the problem on the temperature gauge.

It could be the OP didn't notice a difference in the heater or the temp gauge, or it never noted since the poor fuel economy was more of a glaring issue to the OP.