I've done this with a Nokia N82 before. It doesn't work very well on a track because the direction changes so much so quickly that the gps doesn't sample the distance travelled accurately so the speed is off. It recorded my 17.5 as doing 75km/h one lap and then 55km/h the next
It's better if you have a long straight if you want to do it this way. I strapped the phone to my battery hold down strap on my TC5r with electrical tape and made sure it wasn't touching any belts. The weight of the phone affects the balance of the car but it's not as if you're racing with it attached.
There are relatively cheap radar guns for sale on Amazon that would do a better job of recording your top speed.