Why has my mileage dropped 17 mpg in 3 weeks?

I'd start by putting your tyre pressures at the correct psi ratings. I'm pretty sure 30psi is well under what it should be. Perhaps others could confirm exact pressures for a gen2 Prius?

Also, you should be using Drive Mode not Brake Mode. D is for Drive as in a traditional automatic transmission and is what you should use 95% of the time (and probably 99.5%). B mode is Brake Mode and is an engine braking mode for going down long steep hills. If you forget and leave the car in this mode you will use much more fuel than you should.

There are probably other fine tuning measures to improve your mpg's but my money would be on the tyre pressures being way under at the moment.

 

Tire pressures (increase to at least standard 35/33 if not more like 40/42), new tires, cold weather - all contributing for sure. Also double check that oil level after the car has been sitting level for a while after driving.
Could also be the 12V battery at this point, check out this thread for more:
http://priuschat.com/forums/newbie-forum/73400-weird-stuff-happening-mpgs-dropping-test-battery.html

- D

 

42 in fronts/40 in backs not vice versa

 

Hi brarain,

Most likely what is happening is the car is running as if it was not even a hybrid, as the engine is not warming up enough to provide you 72 F interior temps, without running continuously. In the 15 F and lower temps, in slow and go driving, the 2nd Gen Prius can be challenged to get warm enough. Pick routes where you avoid traffic for the first 10 minutes of a trip, even if this means driving a little farther. This will get the engine hot enough to get the passenger compartment hot enough, for you to be comfortable, through using heat that is the waste from travelling, not heat generated from a stationary engine. The 3rd Generation Prius addresses this issue with an exhaust heat recovery system - which works well.

You can either continue this way, or save some of the heat that would flow out of the engine compartment, into free space, so that that heat ends up in the passenger compartment.

Search for Grill Blocking - specifcally for the 2nd Generation Prius. There is a difference between how to do it for 2nd and 3rd generations of the Prius.

Yes, do pump the tires up too. Most of use have found that 42/40 PSI (F/R) is best for tire life, aquaplaining, handling, and is a very good improvement on the mileage over 30 PSI. 30 PSI is actually so low as to be dangerous. Tire pressures change 1 PSI per 10 degree F. So, having the tires good at 70 F, results in them being 6 PSI low at 10 F.

The Prius wont run the heater, unless you are set for defrosting, until there is some heat in the engine. So, just cranking things up does little. When you run the window defroster, it kicks on auxiliary electric heaters, which require the engine to generate electricity for. The idea of these, is that a little gas is less important than being able to see out of the windows. If you can see out of the windows, and do not need defrost, before the engine is hot, your wasting allot of mileage.


To answer your question outright: Your mileage went down 17 mpg, because the late fall temps in the midwest were extraordinarily warm - 70's F in November, and the early winter temps in the midwest have been record-setting cold (as low as 5 degrees F here a week ago) and snowy. And you did nothing to set the car up for cold winter operation, that most people do when it gets cold. Or the age-ol Engineer's lament - "Mother Nature is a Bxxxh"...

 

make sure your oil is not above the full mark on the dipstick. my mpg's have dropped from 65 to 49 in the last coupla weeks. same thing every year for the last 6 years. take all the good advice above and you'll be fine. all the best!

 

That's a good start.

That could be a problem. If your MFD shows the same drop, then probably less so, but you have a 2007 so you have a GenII. This means you and I are cursed with the bladder petrol storage. In your subaru, and every other car on the market (including the new GenIII Prius's the GenI Prius, and even the non-USDM GenII Prius) there was/is a tank. This tank was made of a rigid material and could hold X gallons/litres. If you always filled it to the top, and you used Y gallons, then you could always put in Y gallons at the pump. Now, this bladder is a different beast. It is like a ziploc bag wrapped in an enigma shrouded in mystery. The capacity changes depending on the temperature, depending on how much is in there already, your altitude, the rate of pumping, and just your Prius' overall mood for the day. So you can never be sure you "topped off" the tank unless you run it dry, and force in 12 gallons. But then even the next fill up, you will never know if you replaced all the gas you burned (resulting in abnormally high mpg, more miles over less gas). Conversly, if you didnt top it off the time before, and now magically you can top it off or just fill it up more, then you will get much worse calculate mpg (less miles over more gas). Rely on the MFD, and subtract about 1-2mpg from what it displays.

Have you reset the gauge since you bought it? It sounds like you have, but you should. For a while, I would do it every fill up. That way you can figure out better driving patterns with an mpg averaged over a shorter distance. 40-43 is reasonable. 50 is definately possible. But then again so it 30 given the right circumstances.

I assume you mean Farenheit. Do you park outside or in a garage?

What are your city trips? A mile to the store, park then a mile back on single lane roads with no stop signs? Or is your "city" a 2 hour commute through stop and go 8 lane interstates?

65mph is about the knee for economy in the Prius, and under 42mph is another good bend to be in for the city.

Flat? I've driven through MO a bit, and didnt see much in the way of hills.

Synthetic is not really important. The weight and fill is though. Use a good 0w20 and make sure your oil is no more than 1/8" to 1/4" from the FULL divit.

So it is probably a 4 year old battery (2011 - 2007). Might be ok, might not be. I would check with a voltmeter just for kicks. If it sat on the lot for a long time with the dealer just letting people in and out and turning on the radio and stuff, then it might not really have been charged/maintained. The battery is a small Japanese battery that most Americans are not used to. Most don't realize that the battery you find in a USDM Corolla is abou 2 times the size as the battery found in a JDM Corolla. Americans like to sit in their cars with the power off I suppose, but the Prius did not go to a USDM sized battery. So what might constitute a small drain on a "regular" battery to you, is a huge drain on a mini-me battery.

Good and good.

Whoa nelly. Those are low for any car. At a minimum, put them up to factory specs. And since it has been 40F to 15F, I guarentee your pressures are lower than that. Mid 40's cold is a good guesstimate of where you want it.

not bad. I could only get 18mpg out of my 1990 legacy sedan. I traded it in for a more efficient civic when gas "spiked" to $1.80/gal shortly after I started driving.

There are two extremes in Prius land. Lead foot race car driver (which sounds like you aren't) and grandma pants where grandma in a walker could pass you. Both are bad. Drive with the Energy Monitor screen for a bit, and see where your power is coming from. All electric is cool, but bad for economy. Also, gliding where no arrows are shown on the MFD is best. So accelerate briskly to speed + 5mph, then let off the gas completely for an instant. Then press with your big toe only on the accelerator (lightly!). This will get you up to speed using mostly the ICE, then coast/glide using just a bit of electricity to keep everything going. The MFD should show no arrows.

He is refering to the 7second startup delay between the car and the gas engine. You do not need to. For a minimum of 51 seconds after start and/or the first complete stop for a couple seconds after start, the Prius is in its warmup stage. The ICE will run constantly and try not to be used. This is to warm it up and reduce emmissions. Remember, the Prius is designed to be lowest on emissions. High fuel economy is just a side effect. So 8 seconds or 51 seconds of waiting doesnt matter, and you dont have to. Sort of implied, but in this warmup mode the car will try to use electric drive instead of gas drive as much as it can to lessen the load on the ICE. It also runs the ICE rich (uses a lot of gas) for the whole time during warmup. Warmup is a function of temperature obviously, and in 15F temperature, it will take a lot longer which you seem to be experiencing. Any time you use electricity, the gas engine has to restore it inefficiently. So the best is to not use electricity. The ideal Prius start, is parked on the top of a hill (that it did not have to climb up ) and then it takes exactly 52 seconds to get from the parking space to the first stop sign all slightly down hill getting regen along the way.

On your shifter. D Mode is Drive. B Mode is Engine brake mode. Only use B mode when going down a mountain and you want to spin the engine as an air pump to burn energy and prevent excess regenerative braking. If the battery gets full, the Prius switches to friction brakes which will heat up and wear out the pads like any other car. Prevent this by limiting the charge to begin with in B Mode. ONLY USE ON MOUNTAINS! I live on a mountain (2000ft vertical change) and I don't even use B Mode 95% of the time.

That also kills your economy. I would just drive without the hvac on. Put on a jacket and drive. And it has been less than 15F around here lately. To keep you warm, the engine must be really warm. To keep an engine warm when it is cold outside, it needs to burn gasoline. Because combustion is so inefficient, the excess power from the petrol turns to heat, which is what is used indirectly to heat you. So the more heat you need, the more energy is needed to be burned, which means more gas used even if the Prius doesnt want to. It is not a linear relationship either. Setting it to 70F instead of 72F saves substantially more energy than setting it from 70F to 68F. But setting it to off and just toughing it saves the most...

If 2 weeks is measured by the MFD without a reset then the mpg drop was instantaneous but the averaging let it slide for a while. New tires will drop mpg 1-2 or more depending on the LRR qualities of the tire for 3000mi to 8000mi after being installed. Changing your oil will also drop mpg for a few hundred miles.

It appears a combination of bad driving habits, poor Prius driving style, cold temperatures, new tires, and new oil are all conspiring against you to lower your mpg. I would expect your mpg to be where it is or lower if you do not change anything.

 

If you started using a fuel with increased ratio of the crappy ethanol in it, you will see a decrease in milage. The more of the crappy ethanol, the less mpg ( and the more your grocery bill will be ).