Sudden MPG drop
 

I bought my FRS in 2013 and for the first few years I was happy getting ~23 mpg almost all city driving. I calculated it myself after every refuel. One day it dropped to ~18 and has been the same ever since. After a couple years of this, I took it to the dealer and they used a scan tool and said everything was in range. I just fueled up yesterday and got 17 MPG on this tank and it's driving me crazy.

Is there an issue that wouldn't show up on the scan tool or something I should look into to figure this out?

Tires are aired to specs. I've gotten all recommended maintenance. Driving habits haven't changed.

Thanks for any suggestions.

Has the intake air temperature (IAT) sensor been cleaned? Also, when was the last time the engine air filter was replaced? Finally, have you switched to a lower octane fuel?

Maybe the traffic in where you're driving has changed significantly. Could be that you accidentally switched to kilometers instead of miles on your odometer.
Could be a culmination of things- low air pressure in the tires, super dirty air filter, along with the oil needing to be changed and your rear passenger side brake caliper dragging just a bit.

Or you are suddenly worse at math

How many miles on the odometer?
Time for new spark plugs?

The IAT sensor has probably only been cleaned if it was done as part of the scheduled maintenance package I bought. But I don't know for sure. Same for the engine air filter. Maybe the dealer has that on file and I can ask. I have always used 91 octane.

No change in traffic. Didn't switch to kilometers. Tires are aired to specs. Oil has been changed regularly. After one maintenance, they marked the air filter as "yellow" meaning it would need to be changed soon. I don't remember if that got done. I'll have to check that out.

Lmao it's hard to routinely mess up miles/gallons, but I'll start double checking.

46k miles on the odometer. Spark plugs were changed a year ago.

Thank you all for responding

Change your air filter! It's cheap, easy, and fun. If that doesn't fix it, spark plugs. You may also want to try bottle of fuel injector cleaner.

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When you change between MPH and KPH only the speedometer changes. All calculations for mileage stay in the original system.

Was the ~23 mpg same during winter and summer ?
I have a noticeable change between summer and winter. Did you check every fueling or from time to time.


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There is a definite difference between winter and summer blend fuel. Here is the average by month for the history of my car. The dips occur consistently in the winter but 90% of my driving is the exact same route. That does not change per month.

Never knew that! Interesting. And I was tired, that would've given them way BETTER mileage

Looks like your average increased from 2013 to 2020. What was the change in 2019 and 2020 increasing the overall average ? Looks very nice :)


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He drives like a...

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Honestly, not really sure but yes, my mileage has increased over the life of the car. The drive train is stock, and other than semi-standard maintenance has had nothing done to it.

The tires have been changed twice (I'm on my third set) but sets one and two are the same tires (see my owner's journal for details).

I expected it eventually to peak as the car went from stiff to running to aging state but so far at over 160K miles, that hasn't happened.

Driving habits haven't changed either, and as mentioned above, since this is my commuter (80 mile around trip per day) nearly all the miles are on the same road, and same time of day. There is only the occasional road trip. On weekends I drive a different car most the time.

Actually I don't. I'm first off the line (never been beat by a soccer mom), take most curves at twice the "recommended speed" and drive manual a good bit of the time.

The secret is my average speed falls right into the sweet spot of around 50MPH.