Asynchronous DC toy motor speeds with same power applied

i have robot follow object left, right or behind. although have used same kind of dc toy motors 'motor driver' move robot 1 of motor moves faster other. have given same speed each 1 of them arduino 'digitalwrite' function. want them move same speed once take same speed value...

please post code , wiring.. it's easier way see what's wrong..   

Arduino Forum > Using Arduino > Motors, Mechanics, and Power (Moderator: fabioc84) > Asynchronous DC toy motor speeds with same power applied

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