The Elise I used to have was always 8% too high. The Evora is somewhere between 5% to also 8% too high than what the Escort 9500ix GPS speed reads. The Elantra seems to be almost exact to the GPS speed readout.My Evora reads fine. It matches my GPS +/- 1 mph.
My Elise speedometer was reading +5 mph higher than the GPS, which is very common.