Cold Weather Fuel Economy

Welcome to the site. There are good reasons why we call it the "Guess Gauge". Do you believe 4 gallons were used because 4 pips disappeared? Bad assumption. Last night at 32F and 1 pip I "filled up" with ~7.5 gallons.

If you are seeking fuel economy advice: Block your grill, install a block heater, check the tire pressure & inflate to at least 42.

On my last tank using an estimate of 1 gallon per pip and the distance on the MFD I was expecting 41mpg calculated. I calculated 52.7 while the MFD showed 52.2. Usually the numbers are close but I've seen 20% differences.

 

It's widely noted that you can't achieve a consistent "full" each time you fill up a 2004-2009 Prius. The tank is also less willing to hold its full capacity in cold weather than in warm- it has a rubber bladder inside the steel tank, so the gas is never exposed to air (which is good for emissions). Believe your trip computer readout for the short-term. If you want to calculate fuel economy with miles and gallons pumped you will have to calculate an average over several tanks. Individual tanks will be all over the place.

It gets better as the car ages. I bought mine in December and it was a bit of a pain to fill, period, during the first winter, with lots of variability and lots of times when the pump would keep stopping way before the thing could be full. The first summer got the rubber tank bladder nice and stretched out and flexible, and it's been no problem since.

One way to estimate how full a given tank-filling was is to watch how many miles you can drive before the first "pip" (block in the bar graph) of the fuel gauge disappears. In hot weather with a good fill I go 120 miles before the fuel gauge budges, and in cold weather with a bad fill I go 40. The gas gauge works fine (BELIEVE it when it tells you to fill up), but every fill-up seems to be different.

 

Highway 52, right? I've been down your way countless times, usually stopping in Rochester to visit the relatives. But during the summer, I sometimes drive down to Fountain or Rushford then bike through Lanesboro.

Never has my MPG ever been even close to that low. My Prius bottoms out at 38 cruising during that winter. My guess is your tires are squishy soft, your oil is overfilled, and your engine air-filter is clogged. Definitely check them, if still concerned. All harm efficiency.

Then like the others have pointed out, don't take individual fillups too seriously. Capacity of "full" can be illusive.
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We got ours in June, and easily averaged around 50 mpg until cold winter hit. The cold really sucks the mpg out of the Prius, as we now stay around 36-38 mpg. Likely the engine is running much longer in the winter, coasting is not as efficient through on snow and slush, the thermos of hot coolant is surely cold so the emissions equipment needs more time to heat up, and the engine simply isn't as "loose". While colder air is denser, it doesn't make up for the other stuff. I'm guessing your very very cold weather sucks the mpg even more.

Also, when we first got it, it would go between 200-240 miles before losing the first pip off full. As others said, you can only go by the readout, not hand calculating because of the bladder.

 

Hi Wil...,

Definately check your tire pressure. If they were at 35 PSI at 70 F, they will be down to 28 at 0 F. The Prius seems to have the best patch, and minimum tire wear at 42 PSI on the front tires, and 40 on the back. I am running 44 / 42 myself. The rule of thumb is 1 PSI of tire pressure per 10 degree F temperature change.

Be careful who you let change your oil. And be sure they use the right weight - 5W30. Keep the oil level below the mark on the stick.

I would also suggest grill blocking the lower grill 3/4s of the way. Start from the passenger side, and apply pipe insulation tubing into the grill slats. This will help the engine get up above 70 C, which is needed for the car to do its hybrid tricks. The transmission is on the driver's side, and needs air flow for cooling.

This assumes your going to be driving below 65 mph in that 5 F weather. If your going faster, you may want to get an add-on gauge for temperature monitoring.

If you have leaves in your air-intake filter, or tubing, this could contribute to the problem too.

 

Hi Stevecaz,

My mileage is at 54.4 mpg right now through 3/4's of this tank. Morning temps have been in the 0 to 12 F range, and evening in the 15 to 30 F range. But only one snow storm - this morning. I have been using the block heater in the mornings below 10 F. And have grill blocking with a temperature gauge - so I can heat up the engine quick, and then avoid getting it too hot later (using the climate control).

 

Was it reset before you started the trip or was it carrying over some previous low values? If it had been idling a lot or used for very short trips in the cold your carryover mileage would be very low dragging down the longer term averages.

But as others have noted you might not have the tires adjusted, the oil may be overfilled, or you might be using defrost/heat on high blower settings and thereby hurting your mileage as well.

Your method won't work for individual tanks. The bladder fill is far too variable for that. Indicated mpg is accurate (or at least very close) while individual fill calculations are not. Over the course of half a dozen fills or more the average manual calculation will be accurate.

 

I had the same kind of jump in my milage when I inflated the tires to 42. Unfortunately when the temperatures are in the zero range I can't find an air pump that works that low which is when I really need it.

 

I always just use my bicycle pump. It gives me a lower back workout, too.

 

My fuel economy tanked in November when it started to get regularly cold in New Hampshire. This was only due to me warming up the car so I don't have to shiver trying to back the car out of the driveway into a dark street around a blind curve.

When I picked up my darling I was regularly getting about 49-50mpg. At it's worst I was getting about 40mpg. Right now thanks to moderated temps, I'm getting 43-44.

 

Hi Zoo... and ben...,

Most of these carryable battery packs have a built in pump. And its a good thing to have around if you do not close the hatch all the way and run down the battery.

Hi All,

For a data point, started the week with a 7 F startup, had a 50 degree day , and now back in the mid 20's F. Finished the week at 58.9 mpg, but then had some errands around town today and ended up at 57.2 mpg. The warm day was one of the worse mileage days. The Prius tends to want to keep the engine running when the weather is warm, and engine temp is above 76 C. With the stops in my route, the car can be in and out of Stage 4 a few times. And each time there are lost gliding oportunities. One of the best drives home was when it was 25 F, on Friday, and got 64.9 mpg for that trip alone. Every gliding oportunity was had and even glided through a few green lights timed very nicely, thank you. On the warm day (50 F) drive home the mileage was only 52.x. Since Tuesday I have been running 80 % lower grill block.

One of my errands this morning was to go to the Sec. of States office for a new plate sticker. For some reason the re-up never appeared in my mailbox. To avoid the rush, I was there at 7:30 am and 20 F. Its about 5 miles from here, so the quick shutdown impacted my mileage. Then I went over to a grocery store about 2 miles from the SofS office. This is on the other end of town. The Grocery store did not open till 9 am. So I left the car in the lot, and got my walk in early. 2 hours later I am out of the store and drove home about 4 miles. Its those short trips that effect cold weather mileage really. For the 23 mile drives back and forth to work, 25 F weather is actually very good.

The reason the Prius wants to idle the engine during what would be glides, when the weather is above about 37 F and the engine is in the 76 to 82 C range during the warmup period is a mystery to me. I am trying to figure out how to stop that from happening. As you can see it s good for another 10 mpg on the 23 mile trip, if it can be avoided. Part of this is the thermostat opening up. The car gets to about 82, then all of a sudden, its back down to 76 C. And that reinitiates a Stage 3b to 4 transistion. If your in a driving enviorement with lots of lights, you can go through that twice. In the colder weather maybe the thermostat is just opening gradually, rather than going full open all at once.

 

You should be able to get in the low 40s mpg in Minnesota. I'm getting about 45 mpg this winter in PA--lots of short trips.

 

I've been tracking my MPG for over three years along with the temperature. Recently I took the records and performed a weighted average of MPG and temperature to make these charts:

For our metric friends:


Now we don't get much sub-freezing weather in North Alabama but I recorded this set of data in January 2008 cruising at 65 mph (0.8% error on those tires: )

TempF TempC density_kg/m3 %_density MPG
15 -9.4 1.34 111.7% 47.5
32 0 1.29 107.5% 49.3
68 20 1.20 100.0% 53.0
85 29 1.17 97.5% 54.4

(Thanks to Ken@Japan who provided the air density data.)

Vehicle aerodynamic drag is proportional to the air density, which is a strong function of temperature. Humidity also has an effect but minor compared to temperature.

Bob Wilson

 

What am I doing wrong?

My tires are inflated 42/40, I buy the best gas, lowest octane available but my mileage is consistently around 44 in warmer months and lately around 38 in colder weather (40s). This is Texas, we don't get that cold. My husband and I both have 2007 models, but his tires aren't inflated like mine. I'm reading people getting 50mpg and greater or not losing a pip until 200 miles and I just shake my head. I usually lose the first pip around 90 miles and rarely over 100. The cars are kept in the garage so they don't get that cold. How on earth are you getting this great mileage? I haven't been able to get over 38 since the weather got colder and they add the methanol to gas. I'm really curious and open to suggestion.

Thanks

Lisa

 

I've been wondering the same thing. In fact, with the arrival of the last cold snap (nearest *major* city is Dallas) my mileage plummeted from nearly 50mpg, to 40. That's a BIG hit...10mpg, representing 20%. Since I live 30-60 miles from the surrounding cities, that means a lot of driving...today I drove over 100 miles.

Most of my driving is highway...I've tried trips with and without the cruise control, steady 55mph, 65mph, 70mph, and "who cares" speed control...all return similar results. While not billiard table flat, our landscape is comprised mostly of flats, with a few small hills. It's not like I'm climbing mountains.

No codes have been thrown, so there's nothing "wrong" with the car, but the man at the Toyota house, today, admitted that 3 others have complained about the same thing *this week alone.* He also admitted that "acceptable" is a range, and maybe I'm at one extreme end of that range.

Something is going on...and Toyota is aware of it.

Chuck

 

In summer temps using a thicker viscosity oil will cause a 10-20% loss in MPGs.
In cold weather the MPG loss will be even more pronounced.

 

Engine runs more to keep the coolant hot. Your cabin heater runs more to warm the cabin, pulling heat out of the coolant. Heater sends cool coolant back to the engine, engine has to start and run to heat the coolant. Engine runs on gas. Gas mileage goes down.
We live in Minnesota. Temps have been -8 to +12F this week. Mileage went from 42 to 37, same route.
You are in the ball park