Fyi you don't need to warm the car up, ....just start the engine, seat belt on & off you go driving sympathetically until the engine's properly up to temperature (not just water temp but oil temp too more importantly).
E.g: Don't over rev the car (try to stick below 2.5-3rpm) for the first 10 miles/10-15 mins of driving.
Op that MPG is very wrong.