Spooky, i was just chatting to someone about this very subject.
Fixed costs are anything which is a cost whether you move or not, ie: insurance, road tax, yard charges if you’re paying somewhere, MOT, leasing cost for the truck if you’re paying it up, age-related depreciation if you own it.
Variable costs are anything which only cost when you’re driving - fuel, servicing, tyres, and a mileage-related depreciation charge or pence-per-mile charge on our lease.
Depending on how you employ drivers that can be either a fixed or variable cost if you employ permanent drivers or use temps.
The other fixed cost which people forget about is your own wages - you need paid!
As an example … if you’re an o/d running one truck then you might be paying say £1000 a month for the truck, £100 for road tax, £200 for insurance, £250 for a yard, plus £2000 for your wages so that’s £3550 per month before you turn a wheel. Say fuel costs £0.60 per mile, it needs serviced every 10,000 miles at a cost of £500 so that’s another 5p per mile, tyres cost £3000 per set and last 50,000 that’s 6p per mile. Total £0.71 per mile.
If you average 20 days work per month then your per-day cost is £177.50, or £19.72 per hour on a 9 hour day. If you do 200 miles a day that adds £0.89 per mile for your fixed costs plus £0.71 per mile for your variable making a total of £1.60 per mile. You need to allow some room to breathe and pay for one-offs such as punctures so you would want to charge maybe 10% on top as contingency, otherwise any unforeseen costs are coming out of the only only optional cost you’ve got - your own wages!
I’ve just plucked some number out of the air here to illustrate the point, and I may have missed some items out. I don’t run any vehicles so I may be way off, but I have been in business for many years and the principles are broadly the same. If you do more miles per day the per-mile cost would come down a bit as you can spread the fixed costs over more miles, similarly less miles would push your cost up. Hope this helps