Average fuel efficiency steadily decreasing

Could be the season change, that imposes a penalty every year. Even if you're hypermiling your guts out, the car's reading the (dropping) ambient temperature, and varying the warm-up regimen accordingly. So both the weather and the car's programming will reduce mpg.

Try dropping the cabin temp down, or shutting the system right off at red lights? That'll help trigger engine shut-down when the car's partially warmed.

Also: how many miles on it?

 

When was the last time you changed engine air filter? All cars that I've own, when mpg dips, 9 out of 10 times the engine air filter was the culprit. Check that first before hittting the panic button.

 

Just a note: op's mpg numbers are more-n-likely miles per imperial gallon. For example 53 mpg (imperial) equates to 44 mpg (US).

Still, that sounds a lot like our summer vs winter numbers, and we're similar (west coast) clime.

 

Measure the 12v battery voltage ; after your next commute power car down, wait for 30 min and measure the voltage, if it's 12.6vdc or greater, then battery appears fine for now. Also wait over night and measure voltage again next morning.

Could be 12v depleting slowly. When I left parking lights on for 4 hours by accident, I turned the engine over and hv battery went from 8 bars to 2 bars within 5 minutes in park, the 12v battery sucked hv battery to be recharged. This was the only time I ever saw hv battery depleted quickly.

 

Some (by no means all) of the failure modes of the 12 volt battery have the inverter trying to recharge it continuously when in Ready. This electrical draw pulls power, so reduces mileage. You may wish to check the battery voltage when sitting over 8 hours but before putting it in Ready. (a zero cost rough estimate is to operate the power windows in ACC, then in Ready. If the speed is WAY different, you need a 12 volt battery) I do not know British brands of batteries.

http://www.amazon.co.uk/Optima-Japanese-Terminals-Reversed-8072-176/dp/B005CU65SO

 

welcome!
there are so many items on prius that can have a small effect.
it's difficult to know for sure how your mpg's are doing if you haven't kept long term tank records.
from tires, brakes and wheel bearings, to fuel, oil, throttle body, maf, egr and everything between. it's a slow process of elimination.
and with such a small drop, it could be anything or everything.
all the best!

 

A good detective does NOT automatically rule ANYTHING out without good evidence.
At this point, you have NO good evidence of anything.

Cold weather uses more fuel. Not much more but the change you noted isn't much either.

At 5 years old, it could be a LOT of things. Many of them just due to tiny changes because of age that add up over time.

If you must obsess over this ~2 MPG change, then test/check EVERYTHING.
Tires, brakes, pcv system, throttle body, spark plugs, exhaust blockage, plugged air filter, battery cooling and read codes to see if any "hidden" ones are set.

Or let a shop give it a good going over.
Or trade it in on a new one.

Or don't worry about it until it gets worse........if it does.
It might recover next spring.

 

While it is a bad result, it does not seem like the symptom of gratuitous charging.

 

12.2 volts is a battery in significant trouble. If multiple days idle is a regular occurrence, I'd invest in a smart charger. That's precisely our situation btw, and I hook it up any day the car's not going to see use, and leave it on (which you can do with no ill effects) till the next use.

 

In your shoes, Id go and buy chevron complete fuel system cleaner concentrated and dump it in the fuel tank before filling up all the way. It's $4.97 at Walmart. Sounds like proper maintenance been done and a dosage of that bottle might help.

Where do you fill up on gas?

 

agree, your 12v needs charging and possible replacement. after that, you can eliminate it from the list of potential problems.

 

No 12.2 likely indicates that there IS a problem but it might not affect your gas mileage.
A healthy, fully charged AGM battery should maintain a resting voltage up around 12.7 volts for WEEKS.
12.2 likely indicates that it is about 50% discharged and that is not normal.

AND.....the second sentence quoted above tends to show that you don't "get" what I said about not automatically ruling out ANYTHING.

Good luck.

 

Mileage on my 2010 Prius was fairly steady 43-44 mpg through Sept 2015, then the average mileage took a noticeable drop, almost 2 mpg, after replacing Bridgestone tires with Michelin Defenders. Replaced the air filter at 46661 and 80126 miles.

 

that sounds right. why would you put on defenders if you don't want to lose mpg's?