Palindromic Odometer

The odometer on a car showed 15, 951 miles.  “Curious” thought the driver, “that’s a palindromic number” (i.e read the same backwards as forwards) “it will be a long time before that happens again”. But 2 hours later, the odometer showed a new palindromic number.  How fast was the car travelling (on average) during those 2 hours?