Today I biked over 8 miles with pokemon go opened but I must say I did not get credit for 13 km worth of Dratini candy. Somehow, I get only 6.4 km worth of candies. I'm not sure actually if biking gives you candy faster than walking. I did some tests on a 0.5 km loop, and walking gave me 0.4 km worth of candies, biking more like 0.2 km in candies. I was biking quite slowly, and I never got the "You're going too fast!" message. Nonetheless, biking seems to give less candies per actual km for some reason.
Curiously, I can get up to 0.4 km/hr worth of candies with the phone stationary, due to GPS drift.