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Re: [Phys-l] Hybrid mileage

I don't drive a hybrid, but I have pondered this question of summer vs winter mpg values for a gas-powered car.
I come up with two (seemingly) competing effects:

1) In summer, the drive wheels typically roll without slipping. In winter, rolling with significant slipping is read by the odometer as an inflated value for the distance traveled and so yields an inflated value for an mpg value calculated from odometer readings. Of course this is an erroneous value of the mpg; the actual miles *travelled* per gallon is less with the slipping.

2.) In winter the engine runs at lower start and warmup temperatures, and for a longer time than in summer. In response, the computer feeds a richer gasoline mixture to the engine, during the (longer) warmup time. And even after warmup the lower temperature of the intake air prompts a richer gasoline mixture from the computer during winter These lower air temperature effects result in a diminished mpg value during the winter.

My experience is that, even using the inflated mpg values given by effect 1), effect 2) wins. Winter mpg readings based on odometer readings are typically less than summer mpg readings.

If you wish to enhance the fictitious 1) effect, run your car on blocks - travel zero miles per gallon but read fantastic mpg values from your odometer ;>)

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)

From: "Robert Yeend" <>
Sent: Tuesday, January 05, 2010 3:16 PM
To: "Forum for Physics Educators" <>
Cc: "Nancy Seese" <>
Subject: [Phys-l] Hybrid mileage

A colleague asked me why it is that she consistently gets 6-8 mpg
less in the winter than summer with her Prius Hybrid. Her mechanic
said that that was "normal," but gave no explanation. Nor can I. She
said that she uses the heater in the winter about as much as the air
conditioner in the summer.


Bob Yeend
Forum for Physics Educators