I fell into this trap when I bought my last block of 5000km, and ended up donating $20 to the Government's coffers.
My sin? I asked at the post office about the ramifications of towing a float behind our diesel-powered Toyota Landcruiser.
As diesel vehicle owners will know, you buy your mileage based on the weight of the vehicle, rounded up. The Landcruiser weighs 2.5 tonnes, thus buying three tonnes easily covers it, allowing for passengers and a load in the back.
However, a tandem-axle double float weighs about one tonne, and two horses would give no change out of an extra tonne. That's an extra two tonnes behind the Landcruiser, making an all-up road-going weight of 4.5 tonnes.
This caused some displeasure, especially given that I was forking out for two extra tonnes over my entire 5000km (at about $10 a tonne), when the truck would only be pulling the float maybe 5 per cent of the time.
I put the same question to the Commercial Vehicle Investigation Unit of the police and got an explanation that will save me money next time I buy miles.
Yes, the diesel Landcruiser does indeed need road miles to cover three tonnes.
However, a float is classified as a light trailer (provided it weighs less than 3.5 tonnes) and therefore does not qualify for road-user charges.
In terms of road-user charges, what is important is the downward pressure the float exerts on the towbar. This should never be more 100 or 120kg with a properly designed float (indeed, any properly designed trailer).
Therefore, as far as road-user charges are concerned, a loaded tandem-axle float probably adds only 100 or so kilograms to the overall weight of the towing vehicle.
In the case of a Landcruiser, if the vehicle weighs 2500kg, add 100kg for the float, and you're still left with 400kg, which should comfortably cover any people and their gear within the three-tonne limit.
Bear in mind that the rating printed on your tow bar, which might be anywhere from one tonne to 3.5 tonnes, refers to the maximum weight it is designed to pull and not the weight exerted downward on the towbar by the trailer or float.