Vehicles & fuel
Fuel costs, economy, charging, tyres and what a journey actually costs.
What a vehicle costs to run, and what a particular journey costs. Fuel and electricity prices are fields rather than published figures, because they move weekly and a page carrying one would be wrong within a fortnight.
The measure worth understanding first is fuel economy, because miles per gallon lies to you. It is not linear: going from 10 to 15 mpg saves twelve times as much fuel over the same distance as going from 40 to 45 does, though both look like a five mpg gain. Litres per 100km does not have that problem, which is why most of the world switched to it.
Fuel & economy
What a tank, a trip or a year of driving costs, and how to compare two cars.
Everything here rests on fuel economy, and fuel economy is the number most likely to mislead you. Miles per gallon is a reciprocal measure: the fuel saved by a five mpg improvement depends entirely on where you started. Ten to fifteen saves twelve times as much as forty to forty-five over the same distance.
- Fuel cost What a journey costs in fuel, from the distance, the economy and the pump price.
- Fuel economy MPG to L/100km and back, with both gallons kept apart.
- Fuel savings What changing to a more efficient vehicle is worth over a year, and over five.
- Range on a tank How far a full tank goes, what it costs to fill, and the range worth planning to.
- MPG from a tank Work out what your vehicle actually returns, from a tank of fuel and the distance it covered.
Electric vehicles
Charging costs, running costs, and honest comparisons against petrol.
The figure that gets left out of most charging calculations is loss. Some of the energy the meter records never reaches the battery — around ten per cent at home, more in the cold or on a slow trickle — and the meter is what you are billed for.
Journeys & mileage
Trip costs, driving time, and what to claim for business miles.
A journey has two costs and people usually only price one. Fuel is the obvious one. The rest — tyres, servicing, depreciation, the share of insurance attributable to the miles — is typically two to three times the fuel, which is why reimbursement rates are so much higher than fuel alone would justify.
- Cost per mile What running a vehicle actually costs per mile, once everything is counted.
- Journey time How long a drive really takes, once the average speed and the stops are honest.
- Mileage claim What business miles come to at your rate, and how much of it the fuel actually was.
- Trip cost Fuel, tolls and parking for a journey, split between everyone in the car.
Tyres & engines
Tyre sizes, speedometer error, engine displacement, power and towing limits.
Tyre notation is three unit systems in five characters: 225/45R17 is 225 millimetres wide, a sidewall 45 per cent of that width, on a 17 inch wheel. Nobody would design it that way now, and it is why changing size without checking the rolling diameter quietly puts the speedometer out.
- Engine displacement Capacity from bore, stroke and cylinder count — plus what the bore-to-stroke ratio says about it.
- Horsepower Convert between hp, kW and PS, and work out the torque behind the power.
- Towing & payload Whether an outfit is inside both weight limits, not just the obvious one.
- Tyre size What a tyre size actually measures, and what changing it does to the speedometer.