Calculating the number of impressions for a moving car with advertisements is tricky. If you check page 6 of this market research report, an agency calculated impressions by adding together all the places the car was driving or parked and the number of cars/pedestrians that passed it at each location
To get a rough estimate, you could also try using this impression calculator. It’s intended for trucks with advertisements, but the concept is the same. You’ll probably just have to lower the final number 30-40% since a car is smaller and won’t be as visible to passerby’s as a semi-truck.
Also note, all of these resources are for impressions in the US. Some factors may be a little different in Nigeria – like the average number of adults per vehicle, or the average number of cars per mile.
Hope this helps!