Google map, Mapquest, and yahoo map, which one to trust?

I have never found any route planning software or website to be 100% accurate. Even if the distances were measured using an odometer on the actual road (which they are not, as far as I know), odometers themselves can vary (depending on tyre pressure and simple mechanical differences) by significant amounts. All route planning databases have to search through millions of possible nodes to choose the optimum route and they all have to add the successive distances, some of which may be small and subject to rounding errors. Different databases will also have different ways of calculating the times based on average speeds for different sections of the route. The best will actually use the speed restrictions and expected traffic delays at particular times of the day but most will use the maximum limits for each section. As a result, the distances may vary depending on the accuracy with which, especially, short distances (in a town, for example) are measured and the driving times may vary even more, depending on the algorithms used. Provided that exactly the same route is chosen, each of the applications should not differ by more than a few percent of the total distance. They are only intended to be - and can ever be - an estimate, anyway, especially when it comes to driving time. Your GPS will probably also use a database if it can pre-calculate routes and this database can be expected to be no better (and hopefully no worse) than any other route planning software.