Rat yourself out and save money on car insurance.

That's the selling point for palm-sized devices, often called "tattletales," that consumers can voluntarily install into a car's diagnostic port to track driving habits such as mileage, speed, time of day and braking intensity.

This month, State Farm Insurance is expanding the program so Minnesota drivers can buckle a technological Big Brother into the passenger seat and potentially pay lower rates. Progressive Insurance offers a similar option nationwide.

"It's a more accurate way to measure auto insurance risks so those customers who are lower risks pay lower rates," said Dick Luedke, a spokesman for State Farm.

The insurers say the devices give drivers control over their rates and encourage good habits. Others wonder if the measures used by the insurers accurately predict risk. One former Minnesota regulator is skeptical and worries about how insurers will use the data they collect.

"One policy-holder's discount is another man's surcharge for not using the device," said former state Attorney General Mike Hatch, who also is a former state Commerce Department commissioner. He questioned whether factors such as speed and mileage accurately reflect risk.

For example, he said a city driver stops more frequently than a rural one. But a rural driver might drive faster, Hatch said, adding that it's a narrow line between reasonable underwriting and red-lining.

Monitoring drivers

State Farm offers some form of the device in 18 states and will offer it in 31 by the end of the month. Luedke emphasized that use of the device is voluntary. Those who enroll get free use of one for a year and an immediate 5 percent discount on their rates.

After the free trial, the State Farm device costs $5 to $10 a month. Drivers can add-on roadside assistance, location monitoring and alerts so a parent might be notified if a teenager ventures somewhere he or she isn't supposed to go.

Progressive's "Snapshot" device is free. Drivers plug in the device and use it for six months to calculate their renewal rate, then return the device to Progressive. The insurer also offers the device free to non-customers for a 30-day trial.

According to Progressive, more than 1 million drivers have signed up since 2008. Drivers save an average of $150 per year, the spokeswoman said.

Progressive's "Snapshot" uses the time of day and speed to calculate the number of miles customers drive as well as how often they hit the brakes.

Allstate Insurance offers a similar device in 10 states and is seeking regulatory approval in Minnesota. Spokeswoman Meghan O'Kelly said that in addition to mileage, the device measures acceleration, braking, speed and time of day, but not location. She said mileage makes up 50 percent of Allstate's risk calculation.

Some 12 percent of new customers have enrolled, she said, with discounts averaging 14 percent.

Neither Progressive nor Allstate track location.

Luedke declined to provide data on State Farm customer usage, but said the average discount is 10 percent. "Maybe you're inspired to drive less knowing the less you drive, the bigger the discount,'' he said.

Drivers' reactions

Some Twin Cities drivers are skeptical about the devices, while others endorse them.

"I wouldn't do it until I knew more about it," said Karen Rivard, who works in downtown Minneapolis.

Tim Holmes of Eagan used the Progressive device and liked it. "I just wanted a break on my insurance," he said, adding that he had an accident on his record.

He gets a 15 percent discount, and his rates have dropped below $100 a month, but Holmes, 29, wasn't sure whether to attribute the better rate to his habits or his increasing age. "I didn't have any fear about my privacy," he said.

Rochelle Olson • 612-673-1747 Twitter: @rochelleolson