After the ECU update, performed in conjunction with my 10K service, I too
thought my mileage had been impacted, possibly because I had read this rumor
in the Yahoo group. I checked my tire pressure to find they had seriously
lowered it (low to mid 30's) in conjunction with the tire rotation. I put
it back to 42/40. Over the next few tanks my mileage was as expected given
the nature of the trips, wind conditions, etc.

I can't honestly attribute my initial and subsequent findings to the ECU
update, tire pressure or paranoia arising out of the rumor but I suspect the
latter.

Posted by mrv@kluge.net on August 24, 2006, 3:20 pm

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NeoPhyte_Rep wrote:

> There's a growing rumor that the last Engine Control Unit program update

> has caused the engine to operate more frequently, thus lowering the

> miles per gallon. I thought I had lost some miles per gallon to the hot

> weather until I heard this. Anyone have any support for this?

Rumor?

It's a learned ISC. Some owners have reported slightly lower MPG,
while the ECUs "relearn their driving habits" (store some data over
time/mileage, to help the tune the vehicle's future performance) after
the 50P ECU upgrade. But it will return to normal in a little while.
Most owners haven't noticed anything after the upgrades, though.

The bigger culprit is that if you live in a hot climate, and are
running the AC a lot, that energy comes from somewhere... Your engine
will come on more often to help power the AC. Whether you have a
Classic (unlikely, as you can't flash the Classic Prius' ECUs) where
the engine has to run to belt-drive the AC compressor, or if you have a
NHW20 (most likely for you) where the engine will come on whenever the
hybrid battery gets low (most likely from powering the electric AC
compressor).

Just remember that the the governmental fuel economy tests (such as
done by the EPA) are done in ideal temperatures (usually in the 70sF)
and do not use the heater or AC...

Posted by NeoPhyte_Rep on August 25, 2006, 10:02 am

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mrv@kluge.net wrote:

> NeoPhyte_Rep wrote:

>> There's a growing rumor that the last Engine Control Unit program update

>> has caused the engine to operate more frequently, thus lowering the

>> miles per gallon. I thought I had lost some miles per gallon to the hot

>> weather until I heard this. Anyone have any support for this?

> Rumor?

> It's a learned ISC.

Please expand the acronym ISC.

> Some owners have reported slightly lower MPG,

> while the ECUs "relearn their driving habits" (store some data over

> time/mileage, to help the tune the vehicle's future performance) after

> the 50P ECU upgrade. But it will return to normal in a little while.

> Most owners haven't noticed anything after the upgrades, though.

Are you saying the Prius' ECU uses more than atmospheric conditions,
accelerator pedal position, and the state of charge of the traction
battery to determine how to control the engine? Do you have a Toyota
document that explains this?

> The bigger culprit is that if you live in a hot climate, and are

> running the AC a lot, that energy comes from somewhere... Your engine

> will come on more often to help power the AC. Whether you have a

> Classic (unlikely, as you can't flash the Classic Prius' ECUs) where

> the engine has to run to belt-drive the AC compressor, or if you have a

> NHW20 (most likely for you) where the engine will come on whenever the

> hybrid battery gets low (most likely from powering the electric AC

> compressor).

> Just remember that the the governmental fuel economy tests (such as

> done by the EPA) are done in ideal temperatures (usually in the 70sF)

> and do not use the heater or AC...

This is not in regards to any governmental fuel rating. This is in
relation to my personal recorded tank fillings and computer readouts. I
record both and both are down about 5 mpg. I consider that
statistically significant.

Posted by Michael Pardee on August 25, 2006, 12:29 pm

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> mrv@kluge.net wrote:

>> Just remember that the the governmental fuel economy tests (such as

>> done by the EPA) are done in ideal temperatures (usually in the 70sF)

>> and do not use the heater or AC...

> This is not in regards to any governmental fuel rating. This is in

> relation to my personal recorded tank fillings and computer readouts. I

> record both and both are down about 5 mpg. I consider that statistically

> significant.

What Michelle is telling us is that the standard does not use heater or A/C,
and for that reason we can expect those to have an impact on our economy
when we do use them. 5 mpg isn't unusual for A/C when the efficiency is so
high initially. It's a matter of fuel used; using a liter of gasoline out of
a tank of gas shows up less on a 20 mpg vehicle than t does on a 50 mpg
vehicle.

Mike

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