Fuel expenses - when is it costing me?

[Mummsnett]

Work pay 45p per mile.
My car has a 53l fuel tank, its a 1.6l engine and according to the spec achieves 78.5 mpg.
How do I work out if its worthwhile driving and claiming expenses? Am I making a profit? The maths confuses me.
Diesel costs 145.9p at the moment.

There are 4.55 litres to a UK gallon
You do 78.5 miles per gallon
Which is 78.5 miles per 4.55 litres
So 4.55 litres costs 664p
Then 78.5 miles costs you 664p
So 1 mile costs you 664/78.5 = 8.46p

These figures are rounded a bit and you will never do 78.5mpg in real life.

The 45p is not just for fuel though, you also have to take in to account tax, MOT, servicing, wear and tear on the car, engine, tyres, higher mileage if you ever try and sell it etc etc. But it looks like 45p would probably give you a profit if you were going to be running the car anyway. If you were buying it specifically for this job then maybe not such a good deal

Thank you for listing that out, incredibly helpful.

45 p a mile is a standard rate that is designed to cover all the costs of running a car including depreciation. Be aware that it's only payable up to 10k miles per year, so if you do higher miles than that, it drops to 25 p a mile.

I think it's a really good deal and can significantly help towards the cost of running a car.

Depending on the car you drive, and yours looks like a very efficient one, fuel will be around a quarter to a third of the 45 p, leaving the rest to cover other costs, some of which will be the same whether you do the business miles or not, eg tax, breakdown cover.

Insurance may be slightly more expensive to have business use added, but usually not much, eg £10-20 pa. Depending on the mileage, servicing, repairs and depreciation may increase, but unless you're doing a lot of miles in an expensive, thirsty car, I think you still come out in front.