accelerate    kunyarutsa, gushiramo
(a car) kwongera umuriro
En-En dictionary 
kunyarutsa (-nyarutse)     v    to accelerate
kwongera (-ongeye)     v    1. to repeat, do again, 2. to give more, 3. to augment, to increase (vt), 4. to accelerate
gushiramo   v    to accelerate