Gearhead Speed bonus calculation

How does the Speed +20% bonus from Gearhead Quality works? Should I sum up this with the Speed Attribute or should I just adjust the speed movement value?

For example: my car has Speed 3. If I choose to improve Speed, should I calculate 3+20% or just adjust my movement speeds with the vehicle to 24/48 m/turn? If I should calculate 3+20%, should I round up the value?

Additionally, Gearhead gives an extra +1 modifier to Speed or Handling that lasts for 1D6 minutes. If I’m using that +20% bonus, should I calculate this after applying this +1 modifier? Or before?