Yes, I was taking depreciation and things like tax and insurance in to account, defrayed across the number of miles driven. There's also servicing and parts, which are partly milage driven and partly time-driven.
The 45p figure from the Inland Revenue seems fairly convincing. And there's these figures from the AA, which range from 30p for a high-milage cheap car to 314p for a low-milage expensive one.
My car is low-ish milage (so more expensive per mile to defray the fixed costs), but was relatively cheap, and has moderate fuel consumption (I get about 30-40mpg depending on what sort of driving).
The 50p figure is in my head from total-budget type calculations I've done in the past, where the total cost is the important figure .There's an argument that for these sort of on-the-margin decisions, the marginal cost is the one to use, rather than the total cost - i.e. only counting how much extra you pay to drive an extra mile (or save by not driving it), ignoring the fixed costs that you have to pay anyway by choosing to have a vehicle at all. That'd be less than 50p - probably more like 25-30p.
Fair points in general, though it's hard to amortise tax and insurance on a per mile basis (insurance does go up with distance, but relatively little compared to overall cost per year). Servicing I guess depends if you do it on a time basis or a distance one (I used to do the former, now the latter since I got a newer car that reminds me). Depreciation is also some mixed formula based on age as well as distance, which probably is specific to the car in question and its average time to various component breakages.
All of which can be roundly ignored (and happily rounded) to some nice, easy to use figure like 50p for the sake of a thought experiment :)
(of course it's fun to consider all the possible nuances before ignoring them)
Yes, insurance is a funny one. I too believe (but have no compelling evidence) that insurance rates vary a lot less with milage than a naive per-mile model would give you. It's possibly because low milage drivers are more likely to have an accident per mile driven than high milage ones because they have less experience; that effect probably tails off for very high milage drivers who have more issues with managing fatigue routinely. Also, the three claims I've ever been directly involved in had my vehicle stationary and parked, which suggests that there's a significant baseline risk of bozos smashing in to you even when you're not driving. As I know to my cost, it's not always possible to recover all the actual costs from the other driver even in cut-and-dried cases of fault.
And as you say, servicing is really complex. I get it serviced and MOTed every year (fixed), and replace various bits and pieces as they wear out (mostly distance).
Having had a closer look at the AA figures I cited above, I think my personal cost is way less than their examples. They have £695 for insurance; my last premium was £140. They have £246 for cost of capital; I always own my car outright, so my cost of capital is the opportunity cost, which is something like 2.5% (paying down mortgage). The thing is probably worth no more than £1,500 at the moment which gives £37.50. (I don't think I actually could replace it for that, but that's the top end of what I could sell it for.) No matter how you draw the depreciation, I can be confident it's less than the £1,274 they give, if for no other reason than that I confidently expect to run it for more than a year before the scrapyard beckons.
So 50p/mile is very much an overestimate of the upper bound for me for total cost. From your mpg figures we can probably infer a 20p/mile lower bound on the marginal cost. 25p/mile and 33p/mile appeal as round numbers to use for ease of mental arithmetic.
I think the only calculation above where that might make a difference is the special trip to the cheaper petrol station. (For the others, it makes the calculation go more clearly the way it went anyway.) At 33p/mile it's about evens to go in the village vs Tesco; at 25p/mile it's 50p cheaper to go to Tesco. Spending 50p to save 15 minutes is £4/h, so on my £10/h metric I should still stick to the village. (Unless I'm near Tesco anyway.)
The 45p figure from the Inland Revenue seems fairly convincing. And there's these figures from the AA, which range from 30p for a high-milage cheap car to 314p for a low-milage expensive one.
My car is low-ish milage (so more expensive per mile to defray the fixed costs), but was relatively cheap, and has moderate fuel consumption (I get about 30-40mpg depending on what sort of driving).
The 50p figure is in my head from total-budget type calculations I've done in the past, where the total cost is the important figure .There's an argument that for these sort of on-the-margin decisions, the marginal cost is the one to use, rather than the total cost - i.e. only counting how much extra you pay to drive an extra mile (or save by not driving it), ignoring the fixed costs that you have to pay anyway by choosing to have a vehicle at all. That'd be less than 50p - probably more like 25-30p.
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All of which can be roundly ignored (and happily rounded) to some nice, easy to use figure like 50p for the sake of a thought experiment :)
(of course it's fun to consider all the possible nuances before ignoring them)
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And as you say, servicing is really complex. I get it serviced and MOTed every year (fixed), and replace various bits and pieces as they wear out (mostly distance).
Having had a closer look at the AA figures I cited above, I think my personal cost is way less than their examples. They have £695 for insurance; my last premium was £140. They have £246 for cost of capital; I always own my car outright, so my cost of capital is the opportunity cost, which is something like 2.5% (paying down mortgage). The thing is probably worth no more than £1,500 at the moment which gives £37.50. (I don't think I actually could replace it for that, but that's the top end of what I could sell it for.) No matter how you draw the depreciation, I can be confident it's less than the £1,274 they give, if for no other reason than that I confidently expect to run it for more than a year before the scrapyard beckons.
So 50p/mile is very much an overestimate of the upper bound for me for total cost. From your mpg figures we can probably infer a 20p/mile lower bound on the marginal cost. 25p/mile and 33p/mile appeal as round numbers to use for ease of mental arithmetic.
I think the only calculation above where that might make a difference is the special trip to the cheaper petrol station. (For the others, it makes the calculation go more clearly the way it went anyway.) At 33p/mile it's about evens to go in the village vs Tesco; at 25p/mile it's 50p cheaper to go to Tesco. Spending 50p to save 15 minutes is £4/h, so on my £10/h metric I should still stick to the village. (Unless I'm near Tesco anyway.)
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