There is no any side effect of driving faster or slower. But however different results of savings are achieved with the OxyHydro unit based on slow or fast (free) driving. The definition of slow driving is basically on heavy traffic or traffic jams, Fast driving would be define as highway driving without any traffic jams.
Of course that is the results of mileage savings. Sometimes of course it is difficult to see based on mileage if the traffic conditions are uncertain but at the end of the month you will get to see your travel expenditure for your fuel has been reduced.
I was just at the workshop this morning. The he did not really calculate the mileage of his car Naza Citra, but however instead of filling petrol every 5 days he is now filling it every 6 days.