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The "green" thermometer (you mean blue, right?) is not a clear indicator that the Prius is in warm-up mode. When you start the car, wait 10 seconds. If the engine is still running, the AC is off, and the multi-display does not show any energy flow (e.g. from the engine to the batteries), the engine is running solely for the purpose of heating its catalytic converters. If I've driven my car within the last 10-20 minutes, this will be a brief period (1-2 minutes). Full warmup time for me is 4-5 minutes in summer, but as much as 10-15 minutes in the winter (if I'm running the heater).

In your case, I'd guess your AC usage was causing extra running of the engine. This loss of stealth mode (we don't have to put that in quotes anymore, because Toyota has begun using the term! :) ) would have the reported effect. Some have reported that the mpg drop caused by running the AC on the highway is less than the mpg drop caused by driving at highway speeds with the windows open. However, AC usage in city driving causes a very noticeable mpg drop. I suggest switching the AC off and opening the windows or turning up the fan.

For more mpg tips, go to and click on "Prius Info" and then "Owner Advice".

Douglas (2002 Silver, Wisconsin)
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