I expect the size of engine and fuel type will make difference to the % knockdown in mpg. More powerful engine wont struggle so much to do the same speeds? You also have to take into account that when carrying multiple bikes, the car is most likely fully loaded too, so that would in itself knock the mpg down somewhat.
We took 3 bikes on roof to Spain last year and noticed about 4-5mpg less. 2.0 ltr petrol Honda FRV
This. I had a conversation recently with a mechanic which basically ended up stating two things:
a) Its all about usuable torque (Nm), not BHP and
b) Often there is a very wide gap between what manufacturers do to bleed from the MPG figures (rolling roads, optimum fuel ratio, optimum atmospheric conditions etc) and what you can achieve on the road. ie, sticking anything outside the car that effects the aerodynamics, or interuppting the airflow even with windows down damages the mpg considerably.
Vis a Vis there is les % difference between particulaly older sports cars and newer models designed with MPG in mind - my beemer for example, wasn't designed to be the most efficient car ever, hence a low % reduction. Stick a bike on a Honda Insight though and watch the efficiency plummit.