American Society for Mechanical Engineers – the 10 second rule has become the 5 second rule based on their research

Virtual Mythbusters by American Society of Mechanical Engineers Florida Section
Contact ASME Florida Section at email address: floridasection@asme.org
The myth we are trying to investigate is the one that says it is better to let your engine idle
because shutting it down and restarting it wastes more gas.
We asked our Florida Section members to perform a simple experiment with their vehicles
equiped with a miles per gallon meter. By using this meter and a special technique, we could
determine the gallons used per minute of idle with only about 5 minutes of actual idle.
The preliminary results with a V6 equipped vehicle and $3/gal gas are as follows:
Idling with no air conditioning on 0.5 gal used in 90 minutes ($1.50 in 90 minutes)
Idling with A/C on an 88 degree F day 0.5 gal used in 60 minutes ($1.50 in 60 minutes)
Restarting uses about the same amount of gasolene as idling for 6 seconds with
the A/C on. See graph of fuel used per minute of idle below.

One researcher reported experiencing about 10 minutes idling at stop lights when
driving 25 miles round trip to work in stop and go traffic.
These stop lights required anywhere from 15 seconds to as much as 1 minute of idling.
If you could conveniently turn off your engine at a stoplight (similar to hybrid vehicles)
and restart it just prior to when the line of cars get moving, you would only save $0.25 of gas per day
assuming you average 10 minutes sitting at stoplights.
If you believed the myth that restarting takes more fuel than idling, the myth is busted.
Our research showed a V6 restart takes about the same fuel as 5 seconds of idling.
We expect a V8 to save even more and a 4 cylinder less.
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