Come tax time, you need a log book which details all of your kilometers and you need all your reciepts for the car.
No capital improvemnts, ie, mags, suspension etc can be claimed, but consumables like petrol, oil, servicing, repairs, tyres can be. Your pertentage of work driving if what you can claim on all the above things.
I work out fuel by estimating my fuel usage per kilometer, I use 14.7 litres per hundred ks (340 to 50 liters) and extrapolate my cost against the average price of fuel during the year.
My log book sits at around 71% driving for work.
You can log a 6 week period and then use that for 5 years. Do a heap of driving and you are laughing.
I use the % method of use, including depreciation. I can then depreciate my car by 22% each year and I can claim the 71% of that depreciation.
Once you figure out all the above, you can work out the cost of your car and how you go against the tax man.
At the end of the day, if you are buying a more expensive car, you are still paying more each month for it, so it a decision you make yourself about what you want to drive and then work the tax as best you can.
BASS OUT