# How can you command a vehicle's odometer to calculate the remaining distance?

My aunt uses an automobile sharing company hereafter abbreviated to ASC (eg: Autoshare or Zipcar), which permits only 200 km daily (excessive mileage is charged); but observe: ASC cares only about the final total mileage, and cares not about the distribution of the mileage over the number of days. Suppose that she rents for 3 days, for a maximum mileage of 600 km.

My aunt must travel from A to B, whose round-trip distance is confirmed as at least 400 km (per Google), but she cannot predict the precise distance (eg: detours, deviations due to road closures).

Objective: She needs to know the remaining mileage in real time (`= 600 - 400 ― 'Current distance travelled'`), to decide whether enough mileage remains to drive elsewhere.

Problem: The ASC's odometer displays only 'Current distance travelled', and so requires my aunt to calculate manually (while driving) whenever she must know the remaining mileage.

• Total distance traveled from zero, or from the last user's amount (e.g. like a normal car)? – phyrfox Oct 18 '15 at 22:16
• @phyrfox I meant `from the last user's amount (e.g. like a normal car)?` – LePressentiment Oct 18 '15 at 23:37
• Is anything wrong with the trip odometer? 600 - trip mileage is not difficult math... – Mooseman Oct 23 '15 at 13:19
• @Mooseman Do you mean that the odometer can calculate this? It is not, but I ask for desire of convenience. – LePressentiment Oct 23 '15 at 13:44