Cars, driving and the school run

What a journey costs, whether the electric one really is cheaper, how much the car quietly takes each month, and what time you need to leave. Put your own economy and fuel price at the top of a note and every answer below follows them.

What a trip costs in fuel

Write the distance, the economy off the trip computer and the price on the pump. Varlig cancels the units and leaves you with money.

calcCopy

# Driving to Cornwall and back

distance = 560 miles gives 560 mi

economy = 48 UK mpg gives 48 UK MPG

pump = £1.42/L gives £1.42/L

fuel = distance / economy in L gives 53.0377166667 L

fuel to 1 dp gives 53 L

trip cost = fuel * pump gives £75.31

cost per mile = trip cost / distance gives £0.1345/mi

# Four of you in the car

split trip cost among 4 gives £18.83

British forecourts sell litres but British cars still quote miles per gallon, so say which gallon you mean: 48 UK mpg is the imperial one on your dashboard, and plain 48 mpg is the smaller US gallon. Dividing miles by miles per gallon leaves a volume, and in L puts it in the units you’ll pay for. See Fuel economy.

Petrol or electric

The two are priced in different units, which is what makes them hard to compare. Turn both into pence a mile and the answer is plain.

calcCopy

# The same 560 miles, three ways

distance = 560 miles gives 560 mi

petrol = (distance / 48 UK mpg in L) * £1.42/L gives £75.31

# Charging at home overnight

home = distance / 4.1 mi/kWh * 7.5p/kWh gives £10.24

# Charging at a motorway rapid

rapid = distance / 4.1 mi/kWh * 79p/kWh gives £107.90

petrol / distance gives £0.1345/mi

home / distance gives £0.01829/mi

rapid / distance gives £0.1927/mi

petrol - home gives £65.07

Miles over miles per kilowatt-hour leaves kilowatt-hours, and a tariff turns those into money, so all three lines have the same shape and the totals can be compared directly. Dividing each one by the distance gives a cost per mile: under two pence charging at home, thirteen and a half pence on petrol, and more than nineteen at a motorway rapid charger. Write the tariff in pence exactly as your bill prints it, and Varlig reads it as money. See Cents and pence.

Filling up and charging up

The small questions at the pump and at the charger.

calcCopy

# A 55-litre tank

55 L * £1.42/L gives £78.10

£60 / £1.42/L gives 42.2535211268 L

55 L * 48 UK mpg in miles gives 580.7188155096 mi

# A 64 kWh battery, charged from 20% to 80%

battery = 64 kWh gives 64 kWh

60% of battery gives 38.4 kWh

38.4 kWh / 50 kW in hours and minutes gives 46 min

38.4 kWh / 7 kW in hours and minutes gives 5 hours 29 min

38.4 kWh * 4.1 mi/kWh gives 157.44 mi

A budget divided by the pump price gives the litres it buys, and a tank times the economy gives the range. Kilowatt-hours over kilowatts is a charging time, which is why the motorway charger takes three quarters of an hour and the one on your drive takes all evening. See Arithmetic with units.

What the car costs a month

Insurance comes once a year, tax once a year, fuel every week. Written as rates, they add up without you converting anything first.

calcCopy

# The real cost of the car

insurance = £620/year gives £620.00/year

tax = £190/year gives £190.00/year

mot = £54/year gives £54.00/year

servicing = £320/year gives £320.00/year

fuel = £58/week gives £58.00/week

fixed = insurance + tax + mot + servicing gives £1,184.00/year

running = fixed + fuel gives £80.69/week

running in £/month gives £350.86/month

running in £/year gives £4,210.29/year

miles = 9,000 miles/year gives 9,000 mi/year

running / miles gives £0.4678/mi

# Against 4,000 miles of taxis at £1.60 a mile

4,000 miles * £1.60/mile gives £6,400.00

Every line is an amount per period, so they add together even though one is weekly and the others are yearly, and in £/month converts the total once at the end. Cost per mile is the figure worth knowing: it turns “should we drive or get the train?” into a sum you can do in your head. See Rent and pay shorthand.

Leaving on time

Work out the driving, add the stops, and see what time you need to be out of the door.

calcCopy

# Friday: 186 miles up the M1

distance = 186 miles gives 186 mi

cruising = 62 mph gives 62 mph

driving = distance / cruising gives 3 hours

stops = 2 * 20 min gives 40 min

on the road = driving + stops gives 3 hours 40 min

arrive by = 6:00pm gives 6:00 PM

Leave at: arrive by - on the road gives 2:20 PM

# Traffic puts the average down to 48 mph

distance / 48 mph in hours and minutes gives 3 hours 53 min

A distance over a speed gives a time, and subtracting it from when you need to be there gives when to set off. Dropping the average speed shows what an hour of roadworks does to the plan, which is the version worth telling everyone else. See Speed, distance and time.

The school run and the commute

Ten minutes twice a day doesn’t sound like much until you add up a term of it.

calcCopy

# Two trips a day, 190 school days

school run = 12 min gives 12 min

each day = school run * 2 gives 24 min

each week = each day * 5 in hours gives 2 hours

each year = each day * 190 in hours gives 76 hours

# The commute, in days of your life a year

commute = 47 min gives 47 min

commute * 2 * 220 in days gives 14.3611111111 days

# Leaving at 8:20 with a 12-minute run

8:20am + school run gives 8:32 AM

Minutes add up into hours and hours into days without you converting anything, and in days at the end says what a year of commuting really comes to. See Adding time to a clock time.

The numbers on the door and in the manual

Tyre pressures, torque settings and loads come in whichever units the sticker happens to use.

calcCopy

32 psi in bar gives 2.2063223338 bar

2.4 bar in psi gives 34.8090570553 psi

110 N·m in lb-ft to 0 dp gives 81 lbf·ft

# A roof box and four bikes against the roof limit

roof limit = 75 kg gives 75 kg

box = 18 kg gives 18 kg

bikes = 4 * 11 kg gives 44 kg

roof limit - box - bikes gives 13 kg

# Towing, in the units the caravan is sold in

1,400 kg in lbs to 0 dp gives 3,086 lb

205/55 R16: 205 mm * 55% in inches gives 4.438976378 inches

Pressures, torques and weights convert like any other unit, and the last line reads a tyre’s sidewall: 205 mm wide, a wall 55% of that, which is the number you want when you’re checking whether a bigger wheel will fit. See Units and conversions.

Make it your own

• Put economy, pump and tariff in the Definition sheet in Settings › Calculations, and every note can price a journey without you typing them again.
• Think in litres per 100 km instead: 48 UK mpg in L/100km gives 5.89 L/100km, and 6.2 L/100km in UK mpg goes back the other way.
• Check a speed limit abroad: 130 km/h in mph gives 80.78 mph, and 50 km/h in mph gives 31.07 mph.
• Split the driving fairly on a long trip: 8 hours 40 min / 2 gives each driver 4 hours 20 min.
• Price the ferry in instalments: £480 in 4 instalments gives £120.00 a time.
• See what the congestion charge costs a year: £15/workday * 46 weeks gives £3,450.00.
• Work out a lease against buying: monthly repayment on £18,000 over 4 years at 6.9% gives the loan payment to compare with. See Loans and mortgages.
• Check a used car’s mileage: 86,000 miles / 7 years gives 12,285.71 mi/year, against a typical 8,000 to 10,000.
• Count down to the MOT: days until 4 June 2027, and first monday of next month to book the service.
• Keep a fuel log a line at a time, tagged #fuel, and put total of #fuel at the bottom of the note. See Tags.
• Work out a sensible following distance: 70 mph in m/s gives 31.29 m/s, so two seconds is a little over 62 metres.