The full-load figure, and why it is an input
A diesel engine at full load burns somewhere in the region of 0.044 to 0.052 gallons per horsepower-hour. That is a range rather than a constant, and where a particular engine sits inside it depends on the engine, its emissions equipment, its condition and how it was tested. A 100 horsepower engine at full load therefore burns roughly 4.4 to 5.2 gallons an hour — a spread of about eighteen percent, which is too wide to bury inside a fixed number.
So it is a field on this page, defaulting to the middle of the band. If you have data for your own engine, or a test report, use that instead. Everything the calculator produces scales directly with this figure, so it is the single input most worth getting right.
Load factor is what turns rating into consumption
The engine rating tells you what the machine can do. Load factor tells you what it is doing. Heavy tillage in hard ground can run an engine near its rating for hours; mowing light grass, raking, transport and most loader work sit far lower, often between a quarter and a half.
The naive model multiplies: 100 horsepower at 65 percent load is 65 horsepower produced, times 0.048, giving 3.12 gallons an hour. That model is wrong, and wrong in a predictable direction, because it assumes an engine producing nothing burns nothing.
The straight line with an offset
Fuel flow against power output is close to a straight line, but the line does not pass through the origin. An engine turning over and producing no useful output still burns fuel — overcoming its own friction, driving its accessories, keeping itself running. That no-load flow is commonly ten to twenty percent of the full-load flow, and it is what makes part-load operation less efficient per unit of work.
So the model here is flow equals the full-load flow times the quantity (no-load share plus the remainder scaled by load factor). For a 100 horsepower engine at 0.048 and a 15 percent no-load share: full-load flow is 4.8 gallons an hour, and at 65 percent load it is 4.8 times (0.15 plus 0.85 times 0.65), which is 3.37 gallons an hour. The naive model said 3.12. The difference is small in gallons and it compounds over a season.
The more interesting output is the consumption per unit of work. At 65 percent load the engine produces 65 horsepower and burns 3.37 gallons, which is 0.0519 gallons per horsepower-hour — about eight percent worse than the full-load figure. Drop further and the penalty grows sharply.
| Load factor | Gallons per hour | Per hp-hr, against full load |
|---|---|---|
| 100% | 4.80 | 1.00 x |
| 75% | 3.78 | 1.05 x |
| 50% | 2.76 | 1.15 x |
| 25% | 1.74 | 1.45 x |
| 15% | 1.33 | 1.85 x |
Read that table twice, because it says two opposite-sounding things at once. Light work costs less per hour and more per unit of work. A big tractor on a light job is cheap to run and expensive to use, and which of those two matters depends entirely on whether the job is priced by the hour or by the acre.
Gallons per hour into dollars per acre
The conversion needs a coverage rate, which is width times speed divided by 8.25, multiplied by field efficiency. Fifteen feet at 5 mph is 9.09 acres an hour theoretically and 7.09 at 78 percent efficiency.
Then fuel per acre is gallons per hour divided by acres per hour: 3.37 over 7.09 is 0.475 gallons an acre, and at four dollars a gallon that is $1.90 an acre of fuel. Notice that field efficiency enters here and not in the burn. Time spent turning at the headland still burns fuel — at a lower load factor, but not at zero — while covering no ground at all. That is the mechanism by which a field with short runs costs more per acre in fuel as well as in hours.
If you want the efficiency figure derived rather than assumed, the field capacity calculator counts the turns and reports it. For the load factor side, the PTO and implement power calculator works out what share of the available power an implement is actually asking for, which is exactly the number this page wants.
What is not in the figure
Fuel cost per acre is one line of an operating cost, and by itself it flatters the machine considerably. Left out here: oil and filters, diesel exhaust fluid where it applies, tyres, repairs, the labour driving it, depreciation, interest and insurance. On many operations those together exceed the fuel, and a per-acre figure built on fuel alone will always look better than the real one.
Also absent, deliberately, is anything about fuel taxes, off-road fuel rules, rebates or the records that go with them. Those vary by jurisdiction, they change, and none of it is arithmetic. The price field takes the number you actually pay and makes no claim about what is inside it. For a different machine and a published burn curve rather than a modelled one, the generator run time and fuel calculator fits a curve through manufacturer figures instead of deriving one from horsepower.
Questions people ask
Where does 0.044 to 0.052 gallons per horsepower-hour come from?
It is the range diesel engines typically occupy at full load in published test data, expressed volumetrically. Individual engines differ within it, and the same engine differs with condition, ambient temperature, altitude and how the figure was measured. Older engines tend toward the higher end, and a well-loaded modern engine toward the lower. Because everything on this page scales with the number, treat the default as a placeholder for a figure you obtain for your own engine rather than as a constant.
Why does part load waste fuel if it burns less of it?
Because two different questions have two different answers. Per hour, a lightly loaded engine burns less — that is straightforward. Per unit of work, it burns more, because the fuel spent simply keeping the engine turning is spread across less output. At full load that overhead is a small share of a large flow; at quarter load it is a large share of a small flow. The table on the results shows both columns side by side so the two effects do not get confused with each other.
How do I estimate my load factor?
The cleanest route is through the implement. If you know what the implement draws and what your tractor can deliver, the ratio is the load factor, which is what the PTO and implement power calculator reports. Failing that, some machines display an engine load percentage directly, and averaging that over a representative stretch of work is better than guessing. As orientation only: heavy draft tillage tends to run high, planting and spraying moderate, and transport, raking and light loader work low.
Should I use engine or PTO horsepower here?
Engine. Fuel is burned by the engine, so the consumption model has to be anchored to what the engine produces, not to what survives the driveline. If the figure you have is a PTO rating, convert it up by the driveline efficiency before entering it — a 65 PTO horsepower machine at 88 percent is about 74 engine horsepower. Entering a PTO figure in the engine field understates the burn by roughly the driveline loss, consistently and invisibly.
Does idling matter over a season?
More than people expect, and the no-load field is exactly the number that quantifies it. At a 15 percent no-load share, a 100 horsepower machine burns about 0.72 gallons an hour producing nothing at all. An hour a day of waiting, warming up and standing at a gate is roughly 180 gallons across a 250 day year, at whatever you pay for it. That is not a large per-hour number, which is precisely why it goes unnoticed until it is added up.