Bike Tax

How can it be right that it costs £20 a year to put road tax on my better half's car and £74 to tax my Bike!!!!!

Her car has a hundred cc and two extra wheels more than the red rocket, now I am not saying put the car tax up as I applaud the fact that the thieving bastards have dropped it even on the dodgy grounds of the environment(power stations, planes diesel buses!), but surely bikes should have lower road tax also if your taxing vehicles on their environmental impact?