So wear and tear doesn't count? Falling value? Insurance and tax. Yes you do have to include them as they are an investment cost which you divide by the miles you do.
Good job I have a disabled train card that reduces the cost by a third. The occasional over 55 specials can reduce it to £12 return. The hour and a half minimum bus journey to the train station is covered by disability bus pass.
I wasn't counting tax and insurance as they cost the same whether or not I drive the car, and as I have no off-street parking I have to pay them regardless.
My car cost me £1300. It is worth £300 as scrap (although as it has new tyres, was serviced last month and has 5 months MOT it will be worth more) - the most it can depreciate is £1000. I will have had it two years when the next MOT is due, so if it fails (which I don't expect it will) it will have depreciated £500 per year. I expect to have covered about 30,000 miles in that time, giving a depreciation of 3p per mile.
If you factor in that depreciation it would break even at 41.4mpg - still lower than the 45mpg I'd expect.
However, if we're counting depreciation on the car, I think it only fair we cover the £3.40 in bus fares it would cost me to get to and from the train station here, making the public transport journey more expensive, thus meaning that even with depreciation I'd only have to average 37.4mpg to make the car cost the same. If I had to spend the same on buses in Grimsby I'd only have to do 34 mpg to make the car cost the same.
Even if I include tax and insurance, and continue to assume that my perfectly functioning car will only have scrap value in 5 months time, the break even point is at 45.8mpg, which is perfectly achievable - and of course the car leaves when I want from outside my house, and goes straight to where I want to go before coming back exactly when I want it to. And as I've said, tax and insurance is basically the cost of owning the car to me, because I have to have them to park it anywhere.
It's a shame public transport isn't more viable in this country, but the simple truth is that since it stopped being treated a 'public good' and instead was treated as a chance to make money (not sure how profit making companies and subsidies quite fit...) it just isn't - hence we spend billions on making more and more roads for cars to queue up on instead of making it more realistically affordable to travel by rail. I want to take my bike up to Scotland for a few days of wild camping and exploring next summer and what with the cost of rail fares and the problems with bikes on trains, plus the time of day I want to leave, it makes much more sense for me to buy another old car (we have one car currently) and drive it there and back and sell it when I get back. That is utterly nuts.