Cheaper petrol - the calculator

Yesterday I talked about working out the cheapest option for petrol, even when the least expensive petrol station is some miles away.

I sat down and did some calculations to try to come up with a formula that could be applied in multiple situations. Here it is.

Petrol price lowest = L
Petrol price nearest = N
Distance difference = D
Fuel efficiency = mpl
Tank capacity = T
Cost of tank nearest = N*T = CTN
Cost of tank cheapest = L*T = CTL
Price per mile = N/mpl = ppm

IF CTN-CTL > ppm*(D*2), then it's more efficient to drive to the cheaper station.
IF CTN-CTL < ppm*(D*2), then it's more efficient to fill up at the nearer station.

Obviously that looks horribly complicated and not exactly quick or intuitive to work out, so I created an Excel calculator that will work it all out for you, and you can download it from here :http://depositfiles.com/files/36xs2tajx

Enjoy :)