why not let it take speed readings once a second, log them in a spreadsheet, and using Excel plot a graph. Then let it interpolate using the other readings... It should be quite accurate.
0-100mph gives a better real life representation of a cars performance though these days as gearing choices for 1st and 2nd can remove a gearchange (before 60mph). IE. My 200sx will just hit 70mph in 2nd.