During the driving portion of the smog test the dynamometer sets a predetermined load on your vehicle's engine. At 15mph this load is twice as high as during the 25mph test. The additional load causes the vehicle to emit more emissions.
In this specific case your Toyota Tercel is producing very different emission results between the two speeds. We'd recommend inspecting the emission sensors which control fuel delivery. It's possible your Toyota's ECU (Engine Control Unit) is unable to determine load; and accordingly adjust air/fuel mixture.
Look into a possible worn MAP (Manifold Absolute Pressure) sensor and/or defective MAF (Mass Air Flow) sensor. Your Toyota's MAF sensor measures the amount of air entering the intake manifold. This sensor must send accurate information to the ECU as well, in order for the ECU to calculate exactly how much fuel to add to the air entering the combustion chambers.
