Used to use my personal car a lot for work.
Mileage claims were at the IRD rate. (I think it was 74c/km back then).
As an example $360 odd for a Auckland - Rotorua return trip. Had an older corolla with had done over 200,000km at the time. Would do that trip on less than $100 of fuel. Never worked out the the full running costs, but for a small car at the end of it's depreciation curve, I think I was around $200 (tax free) in the win with each of those trips.
I was with state for insurance at the time. They were fine with me using the car in this way as long as I wasn't carrying goods, samples etc. They were fine with papers and a laptop which was pretty much what I carried as a consultant.
Company policy was that personal cars only be used as a last resort. (We were meant to either use the pool cars or taxi's within town, and rental cars for out of town trips), but due to the lucrative nature of personal car use we would mostly just make some excuse why those options were impractical, and run our own cars.
We were required to have our own comprehensive insurance, and all damage was our own responsibility in the first instance, but there was company insurance only for use in the event that private insurance failed to respond to a claim.
Economics of it would have been different if I owned a car that with rapid deprecation (say a near new luxury euro car). Also IRD rates are set every year, so don't yet reflect cira $3/L fuel.