The onboard computer mpg calculators provide estimates of your fuel economy - not the actual fuel economy. Usually they have readouts both for "instantaneous" - which is good for looking at how your driving affects mpg, and "average" - which estimates overall since the last reset.
But the only true measure of mpg is by calculation. You fill the tank, set the trip odom to 0, then drive & refill to the same level. Divide miles driven by gals to 2 decimal places. Do this with every fill-up and you'll always know your mpg.
Obviously this isn't possible to do in a test drive. If the prior owner kept a notebook with a record of every fillup and the mpg each time - that would answer your question - but it would only be helpful if your driving pattern was the same as his.