My 2008 Prius dropped from 47mpg to 36mpg.this month. It has 175000 miles, the oil was changed just a month ago, and the only significant change to the vehicle before the mpg drop was that I moved from CO to AZ and spent a couple weeks in CA in between. In CO I would get 47-48mpg in the warm months and 43mpg in the winter generally off of a 20 minute one way commute. I didn't see any changes to the mpg in CA in June, but after arriving in Phoenix about 3 weeks ago I filled it up and from then on I've averaged 36-37mpg. In CO I used 85 octane fuel and in CA and AZ it's been 87. I had the tire pressure checked today and the shop put in about 1psi per tire, but after driving some more I haven't seen much if any improvement. I ran the battery test detailed on another post but it came out exactly as the test stated it should. Another car forum suggested it might be too hot for the battery to function properly (100+ days every day here). Any ideas?
If it's only been 1 tank of fuel so far you may just need to see if it returns to what is normal for your car. It can be many things that effect the fuel efficiency of the Prius as is with all cars. If no codes are showing up or no warning lights it makes it difficult to work out what the issue may be. Things like HV battery health, new roads and driving style, plugs, oil level or even bad tank of fuel.
could be your hybrid battery getting weaker, or just the 12v. have you checked the oil level? are the cooling fan and intake grille clean? in the end, it might just be the heat. get it tinted, and use window shades when parked in the sun.
Three things: You moved to a place that's hotter than h3ll and your driving routine is different from before. Your AC is running MUCH more than before. The HV battery powers the AC so it drains at a MUCH higher rate than in Colorado. The gas engine has to run more often to charge the battery due to the AC drain. A higher temp battery is less efficient and powerful; especially an older, weaker one.
I would also think the gas bladder is becoming more pliable in the heat and gradually allowing an increasing amount of fuel. Within another tank or so it will probably level off and your fuel mileage will increase into the 40s again.
I live in the Phoenix area and have two gen 2s. If you are taking short trips (10 min or less) around town, 35-40 is normal for the summer. If you are taking longer trips (20 minutes or more), your mpgs may possibly concern me. I drive about a 20-minute,17-mile commute with an average speed around 60mph.i usually get about 45 in the summer. On weekends when I'm usually just driving from store to store, I'm into the high 30s.
Thanks for the replies. My commute is 10-15 min at 45mph. On the weekends I'll get on 101 for about 15-20 minutes to go somewhere and the mpgs don't really change. I'll keep an eye on it as the weather cools in the fall.
Do you have your A/C on auto or manual settings? I believe there's a sensor on the dashboard for handling (assisting?) the A/C during auto function. The sheer heat from the Sun + the windscreen induced green house effect might force your A/C to work way harder than it has to. You might want to experiment with using your A/C on manual settings to see if this increases your MPG - I know it works for me; I never use A/C on Auto mode - even on very hot days (though a heatwave in London is nothing compared to "Roasting Arizona" ). iPhone ?
Bisco asked about your 12V battery. I know when mine started to go bad in my 2008 the mpg dropped considerably. This is because the engine is commanded to run more often to maintain a state of charge in the 12V battery that is no longer obtainable. I suspect it has already been replaced at least once because their nominal life is 4-6 years. Better check it or have it load tested at a parts store.
Haven't had a chance to professionally test the battery, but I don't think that's the culprit. Since I started this thread my stop and go city driving has dropped my mpg to around 36. This weekend we drove from Phoenix to Sacramento and as soon as we cleared the city my mileage started to tick upwards. After refueling it went up to 48mpg and stayed there for most of that tank. We refueled again and driving on the highways around Sac it's been holding steady at 44. The temperature in CA all weekend has been roughly 100 degrees so I'm guessing it's just the type of driving rather than heat or battery wear.
It probably is both, but the HV battery rather than the 12 V battery. You may find once the heat returns to more mild temperatures, things will get back to normal, In the climate you have at the moment, short city stop and go will hit the MPG, particularly as the HV battery gets older. But more than likely, plenty more life left in it, so don't worry too much.
When we are in Las Vegas...our 2009 Prius will only get 38-40mpg with the AC running in 110 degree heat...One other thing...you mentioned a change of oil...make sure you only put 3.75 quarts in ...not 4, because an overfill will reduce your mileage, as well...an overfill will muck up the MAF sensor ,too...so keep it under the full mark on the dipstick...Ye ol prius doesnt like extreme heat nor extreme cold...like in indiana in winter...28-35mpg at -5 F cold
I regularly drive 2 2005 prius and the mileage varies on both. One with 190k miles consistently gets 50+ mpg, even in Vegas 110 degree heat. The other one with 110k miles will get low 40s. Still can't figure it out.
