That would depend on how much "assist" the motor puts out... On my system , 350watts max, with a cut out at 32Km/Hr. I can pedal at 20Km/Hr average on level 1, and when I use the assist on my bike on level 4, it goes up to 30Km/Hr on average... Only a 50% gain in speed, but I am only 1/2 as tired, and only getting 1/2 the workout as the motor does the rest...
EDIT; Oh, I forgot to mention,on level 1 assist, I can get 120 Kms on a charge but on level 4 assist, I only get about 40Kms...