Cold Weather Fuel Economy

[willsundog]

I purchased a 08 package 6 with 8,200 miles last Friday (1/23) after driving a Honda Civic hybrid for 4 years.

The temperatures have been cold in MN where I live, holding between 0 and +5 degrees since I purchased the car.

On Monday, I was driving back to the cities and had driven 70 miles (highway driving with cruise) and my mpg estimator was reading 27 mpg. It improved to 31.5 by the time I returned home.

I have been checking actual miles per gallon against the estimators listing. Last night I drove 106 miles and used 4 gallons of gas = actual mpg = 26.5. The mpg estimator listed 40.3. What's up?

I regularly stayed in the low 40's mpg with the Civic hybrid.

Your advice would be appreciated.

[JimN]

Originally Posted by willsundog

I purchased a 08 package 6 with 8,200 miles last Friday (1/23) after driving a Honda Civic hybrid for 4 years.

The temperatures have been cold in MN where I live, holding between 0 and +5 degrees since I purchased the car.

On Monday, I was driving back to the cities and had driven 70 miles (highway driving with cruise) and my mpg estimator was reading 27 mpg. It improved to 31.5 by the time I returned home.

I have been checking actual miles per gallon against the estimators listing. Last night I drove 106 miles and used 4 gallons of gas = actual mpg = 26.5. The mpg estimator listed 40.3. What's up?

I regularly stayed in the low 40's mpg with the Civic hybrid.

Your advice would be appreciated.

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.

[wyounger]

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.

[john1701a]

Originally Posted by willsundog

On Monday, I was driving back to the cities and had driven 70 miles (highway driving with cruise) and my mpg estimator was reading 27 mpg. It improved to 31.5 by the time I returned home.

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.
.

[stevecaz]

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.

[donee]

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.

[donee]

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).

[Shawn Clark]

Originally Posted by willsundog

On Monday, I was driving back to the cities and had driven 70 miles (highway driving with cruise) and my mpg estimator was reading 27 mpg. It improved to 31.5 by the time I returned home.

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.

I have been checking actual miles per gallon against the estimators listing. Last night I drove 106 miles and used 4 gallons of gas = actual mpg = 26.5. The mpg estimator listed 40.3. What's up?

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.

[willsundog]

Thanks for your recommendations. Oil level and air filter were fine. Tires, however, were inflated to 32 pounds. Upon correction, that increased my mpg roughly by 7 mpg. Now getting reliably in the low 40s

Will Sundog

[ZooPrius]

Originally Posted by willsundog

Thanks for your recommendations. Oil level and air filter were fine. Tires, however, were inflated to 32 pounds. Upon correction, that increased my mpg roughly by 7 mpg. Now getting reliably in the low 40s

Will Sundog

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.