The fluc­tu­at­ing weight of the Mars Bar is quite a con­tentious issue here in the UK. Answer­ing a query as to whether econ­o­mists take this into account (and not just price fluc­tu­a­tions) when cal­cu­lat­ing infla­tion using the Retail Prices Index, econ­o­mist Tim Har­ford offers some enter­tain­ing infor­ma­tion regard­ing the Mars Bar unit of account.

The Mars Bar has been wor­thy of scrutiny ever since the late Nico Colch­ester noted in the Finan­cial Times back in 1981 that it was a very sta­ble unit of account. It is a ver­i­ta­ble ingot of basic com­modi­ties (sugar, milk, cocoa) that has kept its value rel­a­tive to the price of other goods such as small cars, which have cost about 20,000 Mars Bars for the past 70 years.

As Colch­ester wrote,

The Mars Bar […] is a long-established bas­ket of sta­ple com­modi­ties (cocoa, veg­etable fats, milk solids, sugar) pack­aged with great con­sis­tency. […] As such it is a reli­able unit of account cer­tainly more reli­able than gold, which is prone to spec­u­la­tion and it pre­serves a remark­ably con­stant real value.

No men­tion of deep-fried Mars Bars, how­ever.