Emission tests can output the fuel consumption for the driving cycle. When related to the work done, this gives the thermal efficiency of the power train. Since the peak efficiency of a gasoline engine may be 35 per cent, but the cycle-average efficiency of a conventional power train may be only about 15 per cent, there is clearly scope for major improvement.

The design of the THS that Toyota evolved for the Prius is evidently robust. Having optimised it for the Japanese driving cycle and road conditions, they have since adapted it for the different driving cycles and road conditions of North America and Europe. This version has lower emissions and fuel consumption on the prescribed cycles, and also more power and performance.

Most systems in the Prius are controlled by 13 Electronic Control Units (ECUs), connected via a network. They are Engine, Hybrid System, Battery, Brakes, Power Steering, Display Screen, Gateway, Meter, Air Conditioner, Body (windows), Theft Deterrent, Double Lock, and Transponder Key. A special purpose computer can be plugged in for diagnostic purposes.

Toyota have had a partnership agreement with the U. S. software house MathWorks for 10 years, and have 400 licences for MATLAB/Simulink (which cost up to US$ 6000 each). When developing the THS and the Prius, Toyota made considerable use of MATLAB/Simulink. This included developing the software for the ECUs.

The U. S. National Renewable Energy Laboratory also used MATLAB/Simulink as the basis for Advisor, which they developed for simulating the emissions and fuel consumption of conventional, electric, and hybrid vehicles. As the first production hybrid vehicle, the Japanese Prius has been extensively tested by various U. S. laboratories, and the resulting data used for validating Advisor.