iphone: KCC Tripmeter. Easy to install & use and its FREE!

Open the app and you will see the first screen, press START at the ZERO TRIP point on your roadbook

If you need to zero the trip, just tap the screen and you will see the second one.

You can now zero the trip OR RESET TO THE DISTANCE IN THE ROADBOOK!

In other words, if you have gone off piste, you can go back to the last Roadbook instruction and set the trip by scrolling the numbers, to the total distance shown there.

Android: DS Speedometer. Again easy to install & use

BUT you will need to make a one off payment of 1,99€ to get rid of the ads. It's impossible to use without doing this!

The instructions are very similar to those above, here are the screens.

Open the app and it will start as soon as you start moving.

The Trip is the small indicator in the speed dial - TOTAL is the larger one underneath.

To reset either the Trip or Total, tap the yellow circle icon and you will see the second screen:   

Again, if you have gone off piste, you can go back to the last Roadbook instruction and set the trip to the total distance shown there by tapping the 3 yellow dots and choosing "Override and entering your distance.