| How much does it really cost to run a car?
Sure you can look up this on the internet (like you're doing now) and get
some numbers. It's a fair bet though that these numbers will simply
reference a source and not explain where they came from. Over the past
6 years I've been recording the costs of running my car - absolutely
everything: fuel use, mileage, maintenance etc., etc. You can now see
how much it actually costs to run a car. Obviously when using these
figures you need to compare like with like, for example, depreciation will
vary considerably with the age and type of car, as will fuel consumption. |
Car
|
| 1994 Peugot 306 sedan SRDT, 1.9 turbo diesel.
 |
| Bought in March 2001 for £3000 with 101000 miles on the clock.
Sold in March 2007 for £300 with 182000 miles on it. |
|
Data recording
|
| Everything that was spent on the car is included in the costs. To
some extent the maintenance costs are reduced as I did most of the basic
servicing myself (a basic service is an oil and filter change every 6000
miles). Major services and repairs (no crash repairs) were done at
local independent garages. Consumables such as tyres are included in
the maintenance costs. Year by year depreciation costs are estimated as the only two definitive figures are
what I actually paid for the car and what I sold it for. The intervening figures
are estimates based on selling prices at the time.
Fuel costs are what I paid at the time to fill up. Mostly in the UK
although there were several trips to Europe. You can see the average
diesel cost over time from the data.
No allowance has been made for inflation in these costs. i.e. "£1
in 2001" was the £1 I paid out, it has not been increased by inflation rates
to 2007 price levels. For information, inflation in the UK has been at
about 3% over this period. |
|
Fuel Consumption
|
| This was recorded simply by noting the mileage and fuel required to fill
the tank up. Its not the most accurate method of obtaining a single
reading as its difficult to fill the tank to the exact same level each time
(full). However, you can see the fill-fill variations on the fuel
consumption charts and if you average these out then the fuel consumption is
very accurate. |
|
Costs
|
| |
2001 |
2002 |
2003 |
2004 |
2005 |
2006 |
overall |
| miles |
16799 |
13600 |
11689 |
15720 |
11089 |
11022 |
80848 |
| depreciation (£) |
800 |
700 |
500 |
300 |
300 |
100 |
2700 |
| tax & insurance (£) |
510 |
389 |
379 |
379 |
377 |
377 |
2411 |
| maintenance (£) |
515 |
749 |
1315 |
800 |
435 |
419 |
4233 |
| fuel (£) |
1294 |
1052 |
913 |
1297 |
918 |
1030 |
6504 |
| Total cost (£) |
3118 |
2890 |
3107 |
2776 |
2031 |
1927 |
15851 |
| pence per mile |
18.57 |
19.77 |
26.58 |
17.66 |
18.32 |
19.61 |
19.61 |
|
|
| The average cost breakdown per year is |
|
average over 6 years (per year) |
 |
| miles |
13474 |
| depreciation (£) |
450 |
| tax & insurance (£) |
402 |
| maintenance (£) |
706 |
| fuel (£) |
1084 |
| Total cost (£) |
2642 |
| pence per mile |
18.57 |
|
|
Fuel Consumption
|
| Here is the fuel consumption (miles per gallon) of the car versus miles. |
 |
| The big dip at 109k miles was caused by fitting a roof rack, loading the
car with climbers and climbing gear and driving down the French motorways at
85 mph to the Alps and back. |
| This is the cost of fuel (diesel) over the years. It is the actual
cost that I paid at the time (pence per litre). The big dips are from
buying fuel in France and Switzerland. |
 |
|
CO2 emissions
|
| So how do you work out your CO2 emissions from fuel consumption? |
| Firstly you need some basic data on fuels*: |
| |
diesel |
petrol |
| density (kg/litre) |
0.84 |
0.72 - 0.76 |
| % carbon by mass |
86.5 |
85.0 - 88.5 |
| Calorific value (MJ/kg) |
42.9 |
41.9 - 44.0 |
|
|
| The average fuel consumption of the car was 45.8 miles per gallon |
| = 19.47 km/litre |
| Now, to get CO2 emission we need to convert km/litre into km/kg.
For this you need the fuel density and it differs for petrol and diesel.
My car was a diesel, so 19.47 km/litre = 16.35 km/kg or 0.061 kg/km. |
| So, each km used 61g of diesel. |
| But diesel is 86.5% carbon so each km releases 52.9g of carbon atoms.
For this calculation I assume that all of the carbon ends up as CO2 rather
than the even more obnoxious pollutant, CO. CO emissions are a tiny
fraction of the CO2 emissions, typically <1 g/km. |
| The atomic weight of carbon is 12 and the atomic weight of oxygen is 16.
To burn that carbon to CO2 requires two oxygen atoms for every carbon atom.
So every CO2 molecule is only 27% carbon. |
| CO2 emissions is therefore 194 g/km. |
| i.e. 45.8 mpg for a diesel car = 194 g/km of CO2 emissions. |
|
| So, for every 10000 miles driven (16100 km), my car emitted 3.1 tonnes
of CO2. |
| Based on my average mileage, (13.5k miles per year), my car's CO2
emissions were 4.2 tonnes per year. |
|
| IF my Peugot 306 was petrol rather than diesel then you need to take
into account the different densities of petrol and diesel. Petrol is
88% the density of diesel so for the same miles per gallon, a petrol
engine's emissions would be 12% less than those of a diesel. Note
however that a diesel engine will do more miles per gallon than an
equivalent petrol engine. |
| Miles per gallon |
CO2 (g/km) diesel |
CO2 (g/km) petrol |
| 20 |
444 |
391 |
| 25 |
355 |
313 |
| 30 |
296 |
261 |
| 35 |
254 |
223 |
| 40 |
222 |
195 |
| 45 |
197 |
174 |
| 50 |
177 |
156 |
| 55 |
163 |
143 |
| 60 |
148 |
131 |
|
| These figures are quite a lot higher than official emissions data for
vehicles. This is because official data are based on laboratory tests
done on a rolling road and whilst not too bad for comparing car A with car B
bear little resemblance to real world fuel consumption. Things that
will increase real world fuel consumption over "official figures" are:
weight carried in car (driver, passengers, fuel, luggage, accessories),
electrical load (lights, wipers, heater, radio, etc, etc), tyre size and
tread, tyre pressures, weather conditions (any wind will increase fuel
consumption as the vector component of a cross wind increases drag).
Driving style also has an enormous effect on fuel consumption as does the
type of use. Based on your car's fuel consumption, you can work out your
own carbon footprint from the table below. |
| Miles per gallon |
CO2 (tonnes/10 000 miles) diesel |
CO2 (tonnes/10 000 miles) petrol |
| 20 |
7.15 |
6.30 |
| 25 |
5.72 |
5.04 |
| 30 |
4.77 |
4.20 |
| 35 |
4.09 |
3.59 |
| 40 |
3.57 |
3.14 |
| 45 |
3.17 |
2.80 |
| 50 |
2.85 |
2.51 |
| 55 |
2.62 |
2.30 |
| 60 |
2.38 |
2.11 |
|
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| * source - Kemp's Engineers Yearbook, 1995 |
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