Chris Woodyard

USA TODAY

General Motors got a nice surprise when it started digging into the data being sent back from a sample of 300 Chevrolet Volt plug-in cars in use in California.

It found that 15% were actually exceeding the car's labelled electric-driving only range. Instead of 35 miles per charge, they are hitting at least 40.

"The fact that most of the folks who purchased the Volt at launch are still enjoying EV range performance on target with when they took delivery is testament to the attention to detail our team paid to delivering on our promise of most people driving all electrically most of the time," says Pam Fletcher, Volt executive chief engineer, in a statement.

Altogether, Volt owners have now logged more than a half-billion miles since going on sale in late 2010. The car is that it stays in touch digitally with GM, allowing the automaker to keep tabs on its driving performance.

As a result, GM knows: