True speed.

Take it to a track and their speed gun will tell you what actual speed you are doing. Tire size and inflation will have some effect on speedometer reading, not sure if there is any way to formulate your speed based on those factors. Car and Driver magazine covered the topic pretty well regarding newer cars and their designed speedometer inaccuracy, which affects European cars the most, with Japanese cars being the second worst offender. The EU requires speedometers to NEVER read LOWER than a cars actual speed, and because car makers know owners are going to mess around with tire sizes, inflation levels, and such that can throw a speedo off, they then have to create a speedo that can err on the high side but never on the low side. Apparently Porsche is one of the worst for inaccurate speedo readings, but in Porsche's defense, they know their owners are more likely than any other vehicle owners to tamper with tire sizes or other modifications, and as such, have to create a speedo that can comply with EU laws while taking into account their owners propensity for altering vehicle specs.