Why mileage gets worse in winter

I think that lower energy content of winter gas is only one of the reasons for lower mpg in winter. Others probably include greater rolling resistance of tires in colder temps, greater drag in wheel bearings, longer warmup of the ICE, and other things I probably failed to think of.

As I get it, BOTH winter and summer gas contain ethanol these days. The difference is in the blend of molecules in the gasoline so that summer gas has lower volatility than winter gas:
Winter vs Summer Gasoline… yes there is a difference « It's just science… and other stuff

Other wrinkles:
- the "10%" label on pumps really says "UP TO 10%"; typical ethanol content these days is around 6%
- in some states the law specifically allows gas retailers to omit any mention that their gas contains ethanol (aren't lobbyists wonderful?), so just because a pump is not labeled ethanol does not necessarily mean that it delivers 100% gas.

Yup, all these apply, along with greater cabin heat, excess heat loss directly off the engine block, drag from precipitation built up on the road, and probably others factors.

We had an all-gas all-year era, followed by ethanol (or other oxygenate blend) in winter only for pollution control, followed by ethanol all year. My comment referred to noticing a substantial summer vs. winter difference back when we still had pure gas year-round, years before the first seasonal oxygenate mandate was imposed in my region.

Who are you addressing?

Litesong. He is spamming sites with the same posts about puregas.

Of course, I ain't spammin'. Without saying it, F8L intimates that I'm lying about my own cars getting much better mpg with 100% pure gasoline.
For some oddball reason, many people who love cars have waltzed right over the 10% ethanol issue, while the ethanol industry pumps more ethanol into our fuel supplies.
To get its best efficiency, ethanol needs ethanol engines with much higher compression ratios than available in gasoline engines. That is why the ethanol industry makes such a big deal about the Indianapolis racing cars.
People who complain about my posts got to have an ethanol industry stick up their rear.

For a perspective from Australia and the benefit of antipodean members in the depths of Winter who might be wondering...
...From my basic research of fuel company websites, there is no Winter petrol (gasoline) in Australia or any change to formulation based on temperature.
FWIW there is Winter Diesel sold in alpine NSW, Victoria and in ACT (where I live). Since it's possible to drive from sea level to the top of Australia in a few hours, there's a warning that filling a diesel on the coast might make starting a diesel engine at the ski resort the next morning a bit tricky.
(Yes, we have Xmas in Summer, Santa wears board shorts and a sun hat and his sleigh is pulled by 6 white kangaroos, called "boomers" for the sound they make as they jump, after whom the Australian Basketball team is named.)

grill block is good for 70 amb temp days or below does that sound correct. Although I have block heater I am also think of a oil pan heater too.................

I have not tried the grill blocking yet. Last winter was our first with a Prius, and we were very happy with how it performed in Maine.

I have discussed block heaters with the dealership personnel. Only a very tiny percentage of their customers get them, and there does not seem to be any trend or feedback on whether they honestly help. If you were in a location where it got seriously cold, I could see it. [Barrows Alaska for example].

There are threads here that discuss the start-up sequence a Prius goes through, engine temp is among the criteria for the engine to feel that it can run at optimum performance. Toyota engineers seem to have spent a great deal of thought into making the Prius a good vehicle, so just allow the vehicle to do what it wants to do.

Where we live, we normally see about two weeks where it hits -20F every night and warms up to maybe -5F each day. For us, that is the 'coolest' part of winter, the second and third weeks of January.

We do not park our Prius inside a walled garage, we have a car port with open sides. So snow does not pile-up on our Prius, but wind freely blows through. We start our Prius everyday. My wife uses it for her commuting to work and back. It rarely sits parked for more than 10 - 12 hours at a time. We have never had any problem starting our Prius. Once on the road, the interior always warms up quickly.

While the MPG does drop in the winter, we have not seen any real difference between 10F driving, -10F driving, and -20F driving.

The Prius is handles amazingly well on ice. So many rivers, lakes, and ponds freeze over; so winter driving routes become much shorter than summer, since we take advantage of water crossings on the ice. The computers that handle individual wheel spin are very nice. I have attempted to get our Prius to 'spin-out' on ice and I have not been able to do it. It is very stable.

I am not sure there would be any benefit to a block heater or an oil pan heater in a Prius. Although I do recommend studded tires October through April.

A couple of hours with the block heater plugged in will raise the coolant temperature by around 30~50 degrees fahreneit. Maybe more in summer, less in winter, when it's fighting an uphill battle against the colder ambient temperature. We use ours year 'round, almost without fail for the first start-up of the day. For 3 seasons at least, the car will auto-stop by the first stop, a block or two away. This time of year, if it's been plugged in for 2 hours, ScanGage will show coolant temp around 110 deg Fahrenheit, from the get go. And it climbs fast.

Do you find a big difference in MPG by keeping the coolant temp around 110F ?

It doesn't stay at 110F, that's basically what the block heater can do, in summer/spring/fall. In dead of winter it would be a bit lower, due to ambient temp's. Typical fully warmed is 190~195.

I believe it has to have a positive impact, especially when you're doing a lot of short trips. Also good for the engine, and get's the cabin comfortable quicker in winter.

I understand that by design it wants to be in it's optimum temperature range.

Whether it gets into that range in the first 20-seconds of run time; or if it takes 60-seconds of run time to get up to temp is the real difference.

When we jump into a cold car in January it does not take long for the car to be up to temp, so I do not see it as an issue.


Do you find a big difference in MPG by keeping the engine pre-heated, and does this equal the expense of running a block heater?

We got the block heater at time of purchase, and use it regularly, so sorry, can't compare. We typically use 2~3 hours, and it's 400 watt. Our current Hydro rate is stepped, either 6.80 or 10.19 (cents canadian) per kWh. I think that means 1000 watts for an hour. With our 2~3 hours at 400 watts, I think that works out to a worst case total of 10 cents, daily. Or 3 bucks monthly.

The big hit with the block heater was professional install through the dealership, $400 all in. The part itelf is somewhere between $60~90, and in hindsight I could have done it, but just didn't want the hastle, with new car purchase.

I can see for guys in really cold climates, where a prolonged warm-up could cause performance issues, and the expense of operating a block-heater would be lesser than the fuel savings. In that circumstance it may be economic to run a block heater. I am not in such a climate, since I am only in Maine.

In our experience, the Prius starts easily when cold, and it warms up quickly. So I am not convinced that operating a block-heater would truly save us. I suspect if it were -40F at night a Prius owner might benefit more from an enclosed garage, rather than being exposed to the weather.

Our Canadian friends might have different experiences. Or I would love to hear from any Prius owner up North where it might get colder, say Alaska.

I have a diesel tractor. Historically they needed block-heaters to start in the winter. But with modern designs even they have gotten away from any need for a block-heater, unless your in cold weather. Fortunately few Americans see cold weather.

ForestBeeKeeper, I don't think that you'll see much difference between 10F, -10F and -20F; they're all very cold. I consider 10°c (50F) to be a cold start.
The impact of an EBH goes beyond fuel saving, which is good because in some places the cost of electricity can be greater than the cost of petrol saved. For example, in the DEFA Technical Handbook (PDF) on pages 4-7 you'll see their test results covering fuel consumption and HC, CO and CO2 emissions as well as wear. DEFA's tests only went down to -20°c (-4F), which wouldn't be unusual in Norway. (-20F is -29°c)

Even in Canberra, where the lowest temperature this year was a mere -6.8°c (20F) every ambulance and fire engine has a coolant heater so that they can floor the pedal as soon as they jump in.
You might not want to floor it, but you might want the heater to work faster, right?

I met a PriusChatter from Sydney (NSW, not NS) who uses as PC EBH (with a 240v to 110v transformer) and they wouldn't know freezing if their tongue was stuck to it! Sydney has never got below 2.2°c (36F).

From what I've read the most effective warmup time period for Toyota's 400W EBH is the first 45 minutes to 90 minutes (essentially warming up the Prius coolant to a prewarmed up temperature about 70F to 100F) and that after 120 minutes there is this diminishing return curve with respect to how much further the EBH can heat up the coolant.

I've notice a drop in my Prius' MPG since the last week of September. Last August, a 15.8 mile commute done at 4am (when all the traffic lights are turned off) got me about 85 mpg. Today (October), doing the same thing gets me about 75 mpg - a 10 mpg drop. The big difference is that the driving temperatue drops from about 65F in August to about 50F in October ( in August I am not grill blocking but in October I've setup the Prius with 100% grill blocking, too). Hypermiling results are very sensitive to thermal loss.

When I did the math for using an engine block heater in my climate in Maine, it didn't pay off (either money-wise or energy-wise). YMMV.

If one is using a Engine block heater to decrease the time until the cabin is warm, it really isn't going to payoff. Get a small electric heater and heat the cabin for a few minutes before driving. 10 minutes at 1000W will do far more to heat the air (and things air touches) in the car, than 3 hours at 400W heating a huge hunk of metal outside, and then trying to use that to heat that same air.

I see, thank you for explaining.

I have had other vehicles that have been hard to start in cool temps, and vehicles that were very slow to warm-up. During that warm-up period they tend to run poorly. I have a truck now that runs very poorly when it's engine has not warmed up. For example, I have a diesel tractor that is extremely hard to start when temps are low. To me, the Prius is a dream to operate, and we have not seen any issues with driving in cool temps.

Since the focus of a Prius is primarily the MPG, I was assuming that there must be some fuel saving feature that would justify operating a block heater.

I just spoke to my wife about this last, she usually heads off to work before I get up. Our driveway is 100 yards long, after each snow storm we commonly will have 4 to 8" of snow on it before I go out and blow it away. She says that the Prius is putting out warm air before she reaches the end of our driveway. So in our minds, it seems to warm-up very quickly.

I am observing a similar decline in mpg due to temp. With an approx. 8F temp drop I am seeing a loss of 2-5mpg loss on my morning commute. Even before i changed my wheels i noticed a mpg hit when temps were lower than 60F.

I also see my afternoon mpg dropping now that midday temps are in the 70s vs. the high 90s. Since rolling resistance can nearly double when going from 100F to 40F it makes perfect sense we would lose mpg.

I wonder how much we could gain by putting electric socks on our tires to pre-warm them.

I've started grill blocking too, 50%, cause temps are in the mid 60s here in north Italy.
My daily commute signs a worse f e , both on urban or highway driving.