If we are going to go all statto then the Retail Price Index and Consumer Price Index both measure price changes in good and services in the UK. They are both based on the same price data but they differ in how prices are combined (into groups or average) and the coverage (i.e. the people whose spendings patterns are used to calculate weights used in different goods/services groups or averages). The CPI is used by the Government for its inflation targets and I think its what is used for index-linked pensions etc. The RPI was traditionally viewed as the measure of rates of inflation that people actually experience in the high street and often produces a higher rate of inflation than the CPI. I wouldn't honestly know how to choose between them for the purposes of calculating inflation of a bike, they both have their merits. Some may say the RPI is more relevant to the individual .
As different types of good/services are affected by different rates of inflation, this may need to be taken into account ("headline" measures are weighted averages combining the specific rates for different goods and services). I have no idea which RPI category would be most relevant to bikes - perhaps vehicle expenditure ? or perhaps (daft as it sounds) expenditure furniture/electrical ? (which are examples of high cost irregular or one off purchases).
Anyone left still awake at this point deserves a medal, or a glass of your favourite drink. Good night.
