WHEN Greig McArthur gets in his Ferrari 360 Modena, he likes to take his iPhone along. He might have to call someone, of course, but it’s certain he’ll use the phone to see how his car is running.

Mr. McArthur, a programmer who lives in Scotland and works there and in New York, likes to use the Dynolicious application on his phone to test the acceleration of his car and, for that matter, anything he happens to be riding in. He says his Ferrari went from 0 to 60 miles an hour in 5.2 seconds and the Q train in Queens reached a maximum speed of 42.4 m.p.h when he rode it about a month ago.

Software like Dynolicious is now appearing as programmers begin to appreciate just what can be done with an iPhone and other advanced cellphones that know where they are and just how quickly they are going someplace else.

Dynolicious tracks the changes of speed with the iPhone’s built-in accelerometer, a silicon-based sensor that measures the motion of a vehicle. The software constantly measures these changes and converts the information into a good estimate of how fast the car is going. Another program, called G-Tac, does much the same thing as Dynolicious. Both G-Tac and Dynolicious also work with iPod Touch models. But many car lovers question the accuracy of the information provided by such software, because the measurements are so small and because any errors can be compounded as the processor adds them up hundreds of times per second. This led Mr. McArthur to buy a special bracket that holds his iPhone horizontally because, he said, his wife’s patience for being told to “hold it correctly” was wearing thin.