Unless driven foot on the floor always or permanently in town that is bad.
My dads mk1 mondeo AUTO 2L does 30mpg always but up to low 40's on a run.
I can't see how anything behind the dashboard could affect mpg, readings yes.
The only true way to determine mpg is to fill full, run it till it's dry, then refill and work it out(on cost of refill), obviously you ned to carry a can of petrol with you for when it runs out.
Most though do a set distance after filling full and use the same station to top up full again then do it that way as it's a lot easier.
tabetha