Estimating a journey's fuel cost
The calculation needs three figures: how far you are travelling, how efficiently your vehicle uses fuel, and what fuel costs. The complication is that efficiency is expressed in several incompatible ways depending on where you are.
The efficiency units
Kilometres per litre and miles per gallon both measure distance per unit of fuel, so higher is better. Litres per 100 kilometres, standard across Europe, measures fuel per unit of distance, so lower is better. Confusing the two directions produces answers wrong by a wide margin.
There is also a genuine trap with miles per gallon: the US gallon is about 3.785 litres while the imperial gallon is about 4.546. The same vehicle rates roughly twenty percent higher in imperial MPG than US MPG, purely because the gallon is larger. Both are offered separately here for that reason.
Why real consumption differs from the rating
Manufacturer figures are measured under standardised test conditions that few journeys resemble. Real-world consumption is commonly ten to twenty percent worse, and considerably worse in some circumstances.
Speed is the dominant factor on longer journeys, because air resistance rises with the square of velocity. Driving at 110 km/h rather than 90 uses substantially more fuel per kilometre, and the time saved is usually smaller than people assume.
City driving is worse than highway driving for internal combustion engines because acceleration from rest is expensive and idling produces no distance at all. Interestingly, this reverses for electric and hybrid vehicles, which recover energy through regenerative braking.
Other meaningful factors are load weight, roof boxes and bike racks which hurt aerodynamics disproportionately, under-inflated tyres, air conditioning, and cold weather, which reduces efficiency notably during the warm-up period.
Getting a better estimate
Rather than using the manufacturer's figure, work out your own. Fill the tank, record the odometer, drive normally, then refill and divide the distance travelled by the fuel added. Doing this over a few tanks gives a figure far closer to your actual driving than any published rating.
Splitting costs
The per-person figure is useful for shared journeys and carpooling. Bear in mind that fuel is not the full cost of driving — tyres, servicing, depreciation and insurance all accrue with distance, and many organisations use a per-kilometre rate several times the fuel cost alone to reflect that.
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Frequently Asked Questions
What is the difference between US and imperial MPG?
The US gallon is about 3.785 litres and the imperial gallon about 4.546. The same vehicle rates roughly twenty percent higher in imperial MPG.
Why is my real consumption worse than the rating?
Manufacturer figures come from standardised tests. Real-world use is commonly ten to twenty percent worse, and speed, load, weather and city driving all make it worse still.
How do I find my actual efficiency?
Fill the tank, note the odometer, drive normally, refill, then divide distance travelled by fuel added. Repeat over a few tanks for a reliable figure.
Does driving slower really save fuel?
Yes, substantially on longer journeys. Air resistance rises with the square of speed, so the difference between 90 and 110 km/h is significant.
Is fuel the full cost of a journey?
No. Tyres, servicing, depreciation and insurance all accrue with distance. Official mileage rates are typically several times the fuel cost alone.