The idea of claiming your car costs for business is appealing. IRD recognises that when there is a mixture of personal and business use, the personal cost should be borne by the taxpayer. Of course, there's no easy way to identify this exactly, but they have developed rules to approximate this. Motor vehicles are the most common large mixed-use item and well developed.
This is not a detailed analysis but should suggest the questions to ask when looking at using a car in business. A small change in factors can make a significant change in costs (as well as administration). It's aimed at SME's with no FBT except for one or two vehicles and no other FBT. There's a few very short videos of the main FBT points on IRD's site
The old rule about companies having to pay FBT has now been relaxed. Any cars bought by closely-help companies from 1 April 2017 can escape FBT. But you have to allocate all vehicle costs (including depreciation, interest and GST) between business and private use (just as sole traders do).
To calculate this requires a log book. You can simplify - down to just for three months every three years. However, if your circumstances change (by more than 20%) you need to repeat the exercise. How you know how much they've changed without repeating the exercise I'm unsure.
There are other ways to reduce FBT, such as days the vehicle is not available for private use (when it's in being repaired or when you travel away from home - both for more than one day). Although unusual (so much so that IRD took a taxpayer to court but hadn't done their homework and lost), it is possible to have a company vehicle that is never available for private use (including between home and work). However, this would be hard to justify if you did not have a vehicle for private use. If you do use it, you must keep clear records.
These rules apply to company-owned vehicles. You also can be reimbursed on a rate per kilometre when you use your own vehicle. The good news is this is no longer limited to 5,000 kilometres per year. The mixed-use is there's no GST involved. The bad news it's no longer a flat rate. The first 14,000 is at a higher rate covering fixed and variable costs. The rest is at a lower rate.
To use this you have to record the odometer reading at the end / start of each financial year. Then it's not simply using the higher rate for the first 14,000 km. The formula assumes part of each portion is business and part is private.