Inflation is the measure of the relative change in prices over a 12-month period.
No, it isn’t. Inflation is a change in the general price level, not relative prices:
An increase in the price of used cars cannot generate or “drive” inflation for a simple reason: it works the other way around. Inflation, which is defined as a continuous and sustained increase in the general price level, causes all individual prices (including wages, interest rates, etc.) to rise more or less proportionately, over and above the change in relative prices caused by changes in demand or supply on specific markets. To the extent that used car prices were partially caused by both inflation and a relative price change, they could not be a cause of inflation.
Relative price changes are when prices change in relation to each other. Inflation is when all prices move.
The problem here is that Spud is using his ignorance of what inflation is to explain inflation to us all.