I think this calculation fails to take into account fixed and variable cost in running a car. The cost of a single journey is fuel and if one is to be pedantic wear and tear accumulated during the Journey.
If not doing weekly runs to collect a rifle I would factor in fuel only.
Fixed costs are costs which cannot be avoided whilst owning the car. Insurance, deprecation, MOT AA/RAC etc. and this is included in the quoted pence per mile figure.
A pedant might argue that deprecation is semi variable in that if you own a car and do very few miles it will depreciate less.
What cannot be avoided is using a whole day up.
So pick a maximum distance you want to travel and just assume your marginal fuel costs for the Journey. Heck your diesel car might thankyou for a motorway journey if you normally only drive around town.
