People arrive here expecting to type in a displacement and read off fuel consumption. It cannot be done, and it is worth saying plainly: fuel use is set by how much work the engine does, not by how big it is. A 6-litre V8 idling in traffic can sip less than a 1-litre three-cylinder held at full throttle up a hill. So this tool asks for the two things that do fix fuel flow — the power the engine actually makes and its BSFC — and then, once you have a burn rate, turns a tank size into run-time and range.
The number that links power to fuel: BSFC
Brake-specific fuel consumption is the fuel mass an engine burns per unit of power per hour, in units like lb/hp-hr. It is the bridge the calculation needs. Multiply the power the engine is actually producing by BSFC to get fuel mass per hour, divide by fuel density (about 6.1 lb per US gallon for gasoline), and you have gallons per hour. A typical petrol engine sits around 0.45–0.55 lb/hp-hr; efficient operating points reach the low 0.40s, rich tunes climb past 0.55.
A worked burn rate
Say an engine is making a steady 100 hp at a BSFC of 0.50. That is 50 lb of fuel per hour, or about 8.2 US gallons — roughly 31 litres per hour. Change the load and the number moves immediately, which is exactly the point: fuel use is a running condition, not a fixed property stamped on the engine.
The part displacement really can't give you: tank size
The second module takes your burn rate and a tank capacity and returns run-time (tank ÷ burn rate) and, if you add a cruise speed, road range. But note what it asks for: the tank capacity, because you have to look that up. Tank size is a vehicle design choice with no fixed relationship to displacement — a big engine can have a small tank and vice versa. Enter 60 litres against that 31 L/hr burn and you get about 1.9 hours; add 55 mph and that is roughly 106 miles of range.
Why displacement is a poor guide to MPG
Road fuel economy depends on vehicle weight, gearing, aerodynamics and how you drive far more than on engine size — which is exactly why agencies measure MPG on standardised cycles rather than deriving it from displacement. Two cars with the identical engine can return very different economy. If you want a car's real-world figure, look it up; if you want an engine's fuel flow at a load, use this tool.
Marine, generator and equipment use
The estimator is just as useful away from the road. For a generator or marine engine running at a steady output, enter that power and its BSFC for run-time and total fuel over a job; the cruise-speed field is optional and only needed when you want road-style range. Everything is shown in both gallons and litres, and both miles and kilometres, so the same result serves whichever units you work in.
References & standards
- US EPA fuel economy — Official US fuel-economy data — why road MPG is measured, not derived from displacement.
Frequently asked questions
Can I work out fuel consumption from displacement?
No. Fuel use follows how much power the engine actually produces and its BSFC, not its size — a big engine at light load can sip less than a small one worked hard.
What is BSFC?
Brake-specific fuel consumption — the fuel mass burned per unit of power per hour, such as lb/hp-hr. It is the missing link that turns a power figure into a fuel-flow figure.
What BSFC should I use for petrol?
Around 0.45–0.55 lb/hp-hr for a typical gasoline engine; efficient operating points reach the low 0.40s and rich tunes climb past 0.55. Pick a point that matches your load.
How does the tank-range part work?
Once you have a burn rate, enter your tank size and the tool divides tank by burn rate for run-time, and multiplies by a cruise speed for road range.
Can displacement tell me my fuel-tank size?
No — tank capacity is a vehicle design choice with no fixed link to displacement. Look it up by year, make, model and trim; the tool asks you to enter it for that reason.
How do I estimate total fuel for a job?
Multiply the power the engine actually makes by BSFC to get fuel mass per hour, divide by fuel density (about 6.1 lb per US gallon), then multiply by hours of operation.
Why is displacement a poor guide to MPG?
Because road economy depends on vehicle weight, gearing, aerodynamics and driving style far more than on engine size. Two cars with the same engine can differ widely.
Does a bigger engine always burn more fuel?
Not at the same load. A large engine loafing can burn less than a small engine held near full throttle, because fuel follows the work done, not the displacement.
What units does the range module give?
Run-time in hours plus, if you add a speed, range in both miles and kilometres, so the same result works for a generator's runtime or a vehicle's touring range.
How do I convert between gallons and litres here?
The tool shows both — pick your tank unit and it reports burn in L/hr and gal/hr and range in miles and km, using 3.7854 litres per US gallon.
Is the fuel figure an estimate or exact?
An estimate keyed to the load and BSFC you enter. Change either and the number moves, which honestly reflects that fuel use is a running condition, not a fixed engine property.
Can I use this for a marine or generator engine?
Yes. Enter the steady power output and BSFC for run-time and total fuel; a cruise speed is optional and only needed for road-style range.
Why divide fuel mass by 6.1?
Because a US gallon of gasoline weighs roughly 6.1 lb, so dividing the per-hour fuel mass by 6.1 converts it into gallons per hour.
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