I am led to believe that the TRE will not actually change the reading of the speedo? - The only way to test this is with a GPS or an accurate radar. Speedo readings are not 100% accurate anyway - at 300km, only a 5% error means you may only be doing an actual speed of 285kmh....and they always read fast, not the other way around

(and speedo error can be higher than 5% as well.......)
Radar or GPS is the only way a true measurement of top speed can be determined.
