From 63mpg to 46

and the winter gas formulation is a bugger. i usually drop 10-20% depending on temp.

 

Our numbers jump around a lot. Here's a bit over a year, calculated mpg in right end (highlighted column). I think you can toss out that 38, maybe some screw up at the pump. I'd like to throw it out, lol. Not really that much difference through the winter. Our winters are pretty mild tho.

 

Looks at my Fuelly and plot the lifetime MPGs and you can clearly see the winter dips.

 

My average mpg has dropped to high 40's last week.

Are you sure it's fully warm?

Mine kicks on because of the heater when it's still warning up - the engine will turn on by itself if the coolant temp isn't enough to satisfy the thermostat setting.

Once I get on the interstate, I have no issues.

 

My 2010 Prius III mpg per tank drops from 68 mpg in the summer to 51 mpg in the winter.
In the winter time - my tire pressure can drop quickly which can cause my MPG to drop too...

The following lowers fuel efficiency on a Prius
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(1) Driving temperatures lower than 50 F ,
(2) trips shorter than 5 miles,
(3) frequent stops and accelerations at the bottom of a steep hill,
(4) speeds over 55 mph,
(5) getting into traffic jams (stop and go city driving less than 15 mph) for longer than 15 minutes , and
(6) using cruise control at speeds under 60 mph.
(7) driving the Prius very fast into tight turns on the road.

The reason these driving scenarios lower the Prius fuel efficiency
=============================================

(1) When outside driving temperature drops below 50 degrees Fahrenheit and driving at speeds under 40 mph the ICE shuts down frequently to conserve on fuel usage - but at colder temperatures the engine and the coolant system can lose thermal energy quickly. When the emission system temperature drops below its peak efficiency temperature range the Prius ICE will startup automatically to reheat the emission system and keep it running hot regardless of whether the car is moving or not - this can lead to a lost of 7 MPG per trip within 5 minutes as measured by a Scangauge II in cold weather. In cold stop and go traffic - this can happen very often - grill blocking can mitigate thermal loss at low speeds but not when traffic jams last longer than 15 minutes. In stop an go traffic - especially in cold weather, the Prius needs to be in power mode so that the ICE is used as often as possible when you need to move forward - because theICE needs to run to keep the emissions system warm. This is not a problem on the highway. When the Prius is constantly going over 50 mph - the ICE is rarely turned off automatically so the emission systems stays hot .... no grill blocking is necessary, and fuel efficiency tends to be pretty good in the winter if you constantly drive about 40 to 50 mph for over one hour.
When the outside temperature drops it can cause the tire pressure to drop or some tire to leak air - when the tire pressure drops the tire's rolling resistance increases and the Prius fuel efficiency can drop - in cold weather, in the winter time, it is vital to a Prius' MPG performance to check and keep the tire pressure up to insure low rolling resistance .

(2) The Prius aggressively runs the ICE for the first five minutes of a cold start to heat up the emissions system to its optimal temperature range at the expense of fuel efficiency. However, once the Prius emission system reaches its optimal temperature (IIRC ScangaugeII coolant temperature FwT >= 177 ) , the Prius fuel efficiency is very very high. Operating a Prius for less than 30 minutes/7 miles at a time reduces its fuel efficiency. Driving the Prius over 45 minutes/12 miles or doing a series of short trip sequentially within minutes of each other can increase the Prius' MPG . In addition, using Toyota's 400watt Engine block heater between 30 minutes to 75 minutes to heat up a Prius' coolant for doing a cold start can mitigate to cold start warmup penalty for a regular Prius Hybrid.

(3) To get a Prius going over 25 mph uphill from a dead stop can often causes the ICE to run in power mode where it is least efficient. If you have a ScangaugeII .. keeping the load(LOD =< 80) under 80 keeps the Prius from guzzling gas - unfortunately when going up hill - this means the Prius might have to crawl uphill unless it has a running start from the bottom of the hill.

(4) The Prius MPG peaks when driven at about 25 mph (about 80 to 90 MPG ) for over one hour and drops linearly as the Prius' speed increases to 80 mph (about 35 mpg) . A Prius' s fuel efficiency is about 30 MPG when driven constantly at about 100 Mph in regular driving conditions. Driving slower helps fuel efficiency on a Prius more than other cars.

(5) In stop and go traffic jams where speeds are less than 15 mph and that last longer than 15 minutes, the Prius' tendency to use the electric motors will quickly deplete the High voltage (HV) battery power reserves and cause the the Prius to automatically and aggressively recharge the Hv battery back up to a 50% level - often while the Prius is standing still. This will cause the Prius' overall MPG performance to drop like a rock. Extended Traffic Jam's lower fuel efficiency can be mitigated by putting the Prius in Power (PWR) throttle mode and relying more on the ICE to move the Prius at low speed via Hypermiling mini pulse and glides - while driving in extended traffic jams - it would easier to do low speed mini pulse and clide for extended Traffic jams if the Prius had a ICE-only (no electric motor) mode - using the electric motors persistently in a traffic jam is less efficient because the kinectic power derived electric motors driven by batteries which were recharged by burning gas to run ICE create electricity is less than the kinetic power derived from an ICE burning gasoline. The challenge with using only gas at low speeds is that it is very easy to accelerate too fast and burn too much energy in stop and go traffic jam scenario with a Prius. A Prius is heavy and will roll at a low speed longer than most cars as long as the regenerative brakes are not activated - often a miniPulse and Glide doesn't need to be too fast for the Prius to *roll* for a very long distance. The best solution is to avoid being in a traffic jam either by changing your route or the time that you are driving.

(6) The Prius is more fuel efficient if regenerative brakes (and the hydraulic brakes) are not used and the Prius speed is allowed to vary when the speed is under 60 mph (in hypermiling this driving technique is called Driving without Brakes or DWB) . It is more fuel efficiency because the energy loss from variable aerodynamic drag when going less than 60 mph is less than the loss of energy from translating kinetic energy to electrical energy via the regenerative braking system. When combined with synchronizing accelerations with down hill grades (gravity assisted accelerations, aka Hypermiling Technique DWL, Driving With Load) - DWB can boost the Prius' FE dramatically over using Cruise control. Cruise control (CC) uses regenerative braking to keep the speed constant but the regenerative brakes only converts 10% to 50% of the braking energy to electrical energy- at speeds lower than 60 mph variable aerodynamic drag from driving at a variable speed from 40 mph to 60 mph is less of an energy loss factor than the energy loss from using regenerative brakes to keep a constant speed - hence at lower speeds from 40 mph to 60 mph - Driving without brakes + Driving with Load (allowing the car to vary its speed rather than using cruise control) on the Superhighway can increase MPG performance significantly as long as speeds stay under 60 mph or you have a tail wind that allows your car to go faster. At speeds over 70 mph aerodynamic drag loss is more significant and regenerative braking occurs less and cruise control starts to make sense. It would be easier to DWL if the Prius had a button to turn off the regenerative braking system to avoid using the regenerative braking system (the only way to turn off the regenerative braking system is to put the transmission into neutral (N) but this also prevents the ICE from powering the wheels too) - I have the Prius HSI display on and work to avoid it going into charging (CHG) side of the bar chart.

(7) The Prius has a very high center of gravity and will lose a significant momentum and will be less stable when turning very fast on very curvy road. When the Prius' computer senses a lost of stablity due to excessive speeds when entering a curvy road it will automatically apply the brakes to slow down the Prius - and the extra energy to accelerate the Prius into the curve is lost as heat when the Prius stability system triggers the braking system. BBC's Top Gear raced a Prius in a curvy closed circuit track to prove this could be done.

 

The engine does not spin up randomly. The ICE is running because a predetermined condition is met. Coolant temperature too low is the most likely culprit.

Pump the tires back up, block the grill & set the climate control lower. Blessed be the @$$ warmers--more efficient to heat your torso than the entire cabin.

I've seen coolant temp drop 5 degrees while waiting for a traffic light. On some trips it took an awful long time to get warm and then it struggled to stay warm.