Absolutely. My ODO is spot on with a GPS speedo but the Speedo is always optimistic. I checked btw the ECU stats and the ECU speed does NOT read the same as the Speedo.
This is a SPECIFIC LEGAL CAVEAT BY TRIUMPH and MOST MFR's. I don't know what teh law is in teh US - But in the EU a Speedo CAN NOT READ LESS THAN TRUE SPEED. As changing tyres (even supposedly of the same size) can result in fluctuations and the law allows +/- 3% to accommodate this - The speedo has to read optimistic. Otherwise anybody can say - Sorry Officer my Speedo said x - even when you know it was really y. Back in the early '80's I escaped a speeding offence in the UK by proving (using a calibrated police speedo) a defective speedo - the dealer and importer got stiff fines and warnings.
Putting an E-Max on the rear will knock it back to 3.5% ish optimistic. A 225/55 Darkside will on paper knock 1.1% off - but in reality it takes more off due to tread depths and carcass differences.