This past spring, I was testing a ScanGauge when I noticed our 2003
Prius would hold the fuel consumption to 0.60 gal/hr regardless of the
accelerator and speed. I already knew the minimum, idle fuel
consumption when 'cold' was 0.30 gal/hr but this fixed fuel
consumption was something new. I soon verified our 2010 Prius has the
Upon further research, there is a Toyota SAE paper that explained
before the catalytic converters reach operating temperature, the Prius
tries to maximize, EV mode while letting the engine run in a minimal
fuel consumption mode. This means the car can accelerate to speeds of
45-50 mph using just traction battery power for the ~45 seconds it
takes for the catalytic converter to become operational. My detailed
write up is here:
The practical effect is maximized by:
1) Don't start car until you are ready to drive, no stationary, cold
idle. So try to park so you can see traffic before having to start the
2) Accelerate to 38-40 mph without pushing the accelerator so the
engine spins faster. Watch the energy flow display and make sure the
power arrow shows from the traction battery. It should take about 30
seconds to reach 38-40 mph.
3) Shift into "N" to coast in minimum fuel consumption mode, 0.30 gal/
hr, for as long as possible.
4) Shift into "D" if you need to stop or slow down for traffic.
5) Until the coolant reaches 70C (you'll need a OBD scanner like a
ScanGauge) shift into "N" whenever practical and safe. Use "D" when
slowing or coming to a stop.
Because of this revelation, I'm changing my warm-up protocol including
parking practices. Initial data indicates this is making a significant
impact but we won't really know until winter weather arrives.
I've notice that if I block the air intake grills that the 2010
Prius will warm up faster and stay warmer for a longer
time after the Prius is turned off. I've also notice that after the
coolant temperature reaches 171F (77C) that it is much easier
to get over an instantaneous 50 mpg energy performance
from the Prius. Using the 1 minute bar FE chart I've
found that from a cold start (when the gasoline engine has been
turned off for more than 4 hours) for the first three to six minutes
of any trip - the Prius normally gets from 15 mpg to 35 mpg -
even with grill blocking, overinflated tires, a HV battery
with > 50% charge, and under ideal driving conditions.
My 2010 Prius will burn 0.60 gallons per hour or more
during those first few minutes to heat up the catalytic
converter, charge the HV battery, and to move the car;
However, If my foot is off the accelerator and the Prius is
coasting downhill (+ charging the battery) during those
first few minutes my Prius will burn as little as .30 gallons
per hour in Drive mode.
During the winter, urban driving environments/routes
(which donot require the ICE to run as often) may
take the Prius up to 35 minutes before the Prius ICE
coolant temperature increases from 108 F to 171 F (77C).
Grill blocking in urban driving environments
can help decrease the time the Prius engine needs to
reach 77C signficantly - especially when the driving
temperature drops below 60F and for shorter trip (<4 miles) .
For longer trips in highway driving environments (>50mph)
the Prius will jump from 15C(60F) to 77C(171F) within a few
minutes and grill blocking is unnecessary.
During winter, during long traffic lights when the Prius might
have a 3 to 5 minute idle-stop ice cycle the coolant temperature
can drop signficantly. If the ICE coolant temperatures drop
too low - while waiting in traffic - the Prius ICE will automatically
turn on just to keep the engine coolant warm and the MPGs
will drop signficantly lower. This happen last winter when
my Prius got stuck in a snow blizzard induced traffic
jam and the Prius kept repeatedly turning on the ICE to keep
the coolant warm while it was stuck for over 40 minutes
subfreezing temperatures. Using the windshield defroster
or the heater accelerated the drop of coolant temperature.
For that particular 16 mile commute route the Prius
got only 22 mpg ( when normally during the winter
time the Prius would gets about 50 mpg on this
route when its not stuck in traffic. So this represented
a 56% drop in FE, Driving temperatures are crucial
factor in achieving high MPGs ; in the summer time,
on this same 16 mile commute route, the Prius can
get about from 60 to 70 mpg! )
2010 Toyota Prius type 3, Blue Ribbon/Dark Grey, oem floormats
Yokohama Avid S33 (front 50 psi/ rear 48 psi)
grill blocking (100% under 60F, 50% under 80F, 0 % over 80F)
ScangaugeII (FwT, SoC, RPM, GPH)
Odeometer +16000 miles/ 60 mpg
I'm finding I can achieve 30-35+ MPG and often up to 50 MPG within the
first 45-50 seconds by exploiting the catalytic converter warm-up
interval. After the catalytic converters light off, mileage is
dependent upon route and keeping the speed up, 35+ mph. Both Prius are
responding well to this new information.
Our 2010 Prius catalytic convert comes on in less than a minute. That
is the only window for aggressive use of traction battery energy to
get the kinetic energy up. Then try to use higher speeds so the
battery recharge overhead is a fraction of vehicle power.
Shifting into "N" as it is easy. I choose to drive both Prius as if
they have manual transmissions and it works.