Truro to Penzance via the A390 and A30 is 44 miles. Sainsburys in Pz are selling diesel at £1.05 per litre which equates to £4.67 per gallon. My Golf will easily manage 45 to the gallon. You do the maths.
That means the fuel cost is approximately £10, not £5 since you presumably wish to get back again, and the round trip will be 88 miles. Unless I have misread something in your earlier post you compared the cost of a return rail fare with the outbound only fuel cost for driving.