Fuel Tank Range Calculator
Enter the tank size and what the vehicle returns, and this gives the range on a full tank — along with the shorter figure you should actually plan around.
Range on a tank
448miles
- Before the reserve
- 403 miles
- Cost to fill
- $47.60
- Cost per 100 miles
- $10.63
- Fuel per 100 miles
- 3.13 gallons
- Leaving a 10% reserve gives a working range of about 403 miles, which is the figure worth planning to.
- The low fuel light usually comes on with around a tenth of the tank left, and the range readout on the dashboard is a guess from recent driving rather than a measurement.
- Running a tank down to nothing repeatedly is hard on a fuel pump, which is cooled by the fuel around it.
About tank range
Range is the tank divided by the rate the tank empties at. That is the whole calculation, and it gives a theoretical maximum that assumes you are willing to arrive with nothing left.
Nobody drives that way, which is why the reserve allowance is here. Holding back ten per cent turns a 448 mile theoretical range into a 403 mile working one, and that is the figure worth planning a leg around.
The dashboard range readout is not a measurement. It takes the fuel it believes is in the tank and applies the economy from the last stretch of driving, so it climbs after a run downhill and collapses when you join a motorway. It is a reasonable guide over the next few miles and a poor one for deciding whether to pass a fuel station.
It is also worth knowing that the low fuel warning is set conservatively — typically with around a tenth of the tank still in it. That is a safety margin rather than an invitation, and repeatedly running down into it is hard on a fuel pump, which relies on the fuel around it for cooling.
What it works out
- Range on a full tank, and a working range after a reserve
- Cost to fill from empty
- Fuel and cost per 100 miles or kilometres
- Any vehicle, in miles or kilometres
The formula
Range = Tank size × Economy Working range = Range × (1 − Reserve)
A 14 gallon tank at 32 mpg carries 448 miles of fuel. Keeping a tenth back leaves a working range of 403 miles, which is the number to plan the next stop around.
On metric the tank is divided by the consumption instead, then scaled: a 53 litre tank at 7.35 L/100km gives 721 kilometres, because litres per 100km counts fuel per distance rather than distance per fuel.
Filling that tank from empty costs $47.60, and the fuel works out at $10.63 per hundred miles. The per-hundred figure is the useful one for budgeting a long trip, because it does not depend on the size of the tank at all.
The economy figure carries all the uncertainty here. Tank sizes are fixed and quoted accurately; economy varies by a fifth or more between a cold winter commute and a summer motorway run, and the range moves with it in exact proportion.
- Tank size
- The usable capacity from the handbook. Most tanks will not take quite their quoted volume.
- Economy
- What the vehicle returns in the driving you are about to do, not its best figure.
- Reserve
- How much of the tank you would rather not spend. Ten per cent is a sensible default.
A worked example
A 14 gallon tank in a car returning 32 mpg, with fuel at $3.40 a gallon.
That works out to 448 miles.
- Before the reserve
- 403 miles
- Cost to fill
- $47.60
- Cost per 100 miles
- $10.63
- Fuel per 100 miles
- 3.13 gallons
Questions
How far can I drive on a full tank?
Multiply the tank size by the miles per gallon. A 14 gallon tank at 32 mpg is 448 miles in theory, and around 403 if you keep a tenth in reserve.
How accurate is the dashboard range readout?
It is an estimate built from recent economy, so it moves as your driving does. It is reasonable over the next few miles and unreliable for deciding whether to skip a fuel station a hundred miles ahead.
How much fuel is left when the light comes on?
Usually around a tenth of the tank, though it varies by manufacturer and the handbook will say. On a 14 gallon tank that is roughly 1.4 gallons — about 45 miles at 32 mpg.
Is it bad to run the tank low?
Doing it regularly is. The fuel pump sits in the tank and is cooled by the fuel around it, so running consistently near empty shortens its life. Sediment sits at the bottom of the tank as well.
Why does my tank never take its full quoted size?
Because the quoted figure includes fuel below the pickup and space the filler neck needs, and because the pump cuts off before the tank is genuinely full. Taking twelve gallons into a fourteen gallon tank is normal.
Does a fuller tank reduce fuel economy?
Only marginally. Fifty litres of petrol weighs about 37kg, which is under three per cent of a typical car and less than a passenger. It is not worth running low to save weight.
How do I work out cost per 100 miles?
Divide 100 by the mpg to get the gallons, then multiply by the price. At 32 mpg and $3.40 a gallon that is 3.13 gallons, or $10.63 per hundred miles.
Does the range change with a full car?
Yes, and with a roof box more than with passengers. Weight costs most in town driving where you accelerate often; drag costs most at motorway speed. A loaded car on a long trip can easily lose ten per cent of its range.