How does static electricity dissipate before a gas nozzle comes in contact with a car?

Dear Car Talk

Dear Car Talk | Jun 01, 2000

Dear Tom and Ray:

How does the static electricity that builds up on a moving car get dissipated before the gas-pump nozzle touches the fill pipe and causes a spark? Just wondering. -- Lawrence

TOM: Static electricity is dissipated through the car's tires, Lawrence. Although not as well as it used to be.

RAY: In the past 10 years or so, tire manufacturers have been developing "low rolling resistance" tires that improve gas mileage by lowering, what? Rolling resistance! And to lower rolling friction or resistance, they use LESS of a substance called "carbon black," which plays a key role in dissipating static electricity.

TOM: But even with the new tires, the vast majority of the static electricity that cars generate is still dissipated through the tires as the car rolls along.

RAY: The bigger danger is actually from PEOPLE. If it was a dry, cool day and you started filling up your car, then slid back into the driver's seat and came out again to remove the nozzle, your touching the nozzle could cause a spark that could ignite the gasoline fumes and cause a flash fire.

TOM: The American Petroleum Institute has recently sent out a warning about this very issue. You can read it at our Web site, the Car Talk section of, by going to the "Got a Car" section and clicking on the "Press Release Center."

RAY: It also reissued some other important warnings: Don't leave the car running while you fill it up; don't fill a gas can unless it's on the ground where it can dissipate static electricity, and don't smoke any El Productos while you're refueling. Follow these simple rules, and you should be able to avoid blowing yourself up at the pump, Lawrence.

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