Whats a "cheap" solution for tracking speed of my RC car ?

I see a lot of people on YouTube have GPS speedometer units, but they seem to go for $150 each, so was looking for a cheap alternative. (Not willing to spend 1/3 of what I spent on the entire setup :) )

I do have a couple of spare old Android phones, however they seem to take time to acquire the GPS signal and give an accurate reading using a speedometer app - and usually by then Ive run out of "road" ;)

Any ideas/decent apps I could look at ? :)

TIA