Calculator 01 / Fuel & Fluids
Fuel & Fluids Calculator
Fuel, DEF and coolant are three different ratios: fuel comes from distance and economy, DEF from fuel burned, coolant from system capacity. Trip fuel cost, DEF consumption, and coolant mix ratio — three quick fluid calculations in one place.
How this calculator works
Units below default automatically — Metric for non-English pages (or if your device's language is set to one we support), US customary otherwise — and convert automatically if you switch the toggle above the form. Changing language resets to that language's default unless you toggle again.
DEF (Diesel Exhaust Fluid) dosing rate varies by engine, duty cycle, and how hard the truck is worked — 2–3% of diesel volume is a commonly cited working range for modern SCR-equipped diesels, so treat this as a planning estimate rather than an exact figure for your specific engine. Coolant mix ratio follows standard antifreeze/water dilution — a 50/50 mix is the typical factory default, with higher coolant percentages used for extreme cold climates up to a manufacturer-specified maximum (usually around 70%, since pure antifreeze actually transfers heat worse than a proper mix).
Worked examples: one from each tab
Trip fuel. 450 miles at 6.5 MPG, diesel at $3.85:
- Fuel needed = 450 ÷ 6.5 = 69.23 gal
- Cost = 69.23 × 3.85 = $266.54
DEF. 150 gal of diesel at a 3% dosing rate = 4.5 gal of DEF.
Coolant. A 16 qt system at 50/50 = 8 qt concentrate and 8 qt water.
Two-stroke. 1 gal of petrol at 50:1 = 128 ÷ 50 = 2.56 oz of oil.
MPG and L/100 km are reciprocals, not a scale
Miles per gallon counts distance per unit of fuel; litres per 100 km counts fuel per unit of distance. One goes up as the other goes down, so they cannot be converted with a simple factor — the relationship is 235.215 ÷ MPG.
| MPG (US) | L/100 km | Typically |
|---|---|---|
| 5.0 | 47.04 | Heavy haul, mountains, or a badly specced truck |
| 5.5 | 42.77 | Loaded Class 8, mixed terrain |
| 6.0 | 39.20 | Loaded Class 8, reasonable route |
| 6.5 | 36.19 | The calculator's default |
| 7.0 | 33.60 | Well-specced aerodynamic tractor |
| 8.0 | 29.40 | Light load or empty running |
| 10.0 | 23.52 | Medium-duty or a light truck |
Because the relationship is reciprocal, a fixed gain in MPG is worth far more at the bottom of the table than at the top. Going from 5.0 to 6.0 MPG saves 7.84 L/100 km; going from 7.0 to 8.0 saves only 4.20. Note also that a US gallon is 3.785 L and an imperial gallon is 4.546 L, so a UK "MPG" figure is about 20% higher for identical consumption.
DEF consumption
Diesel exhaust fluid is metered as a percentage of diesel burned. The rate depends on the engine, its emissions calibration and how hard it is worked — these are planning averages, not a dosing specification, and your dash gauge is the authority on your own truck.
| Rate | DEF per 150 gal diesel | Per 1,000 gal | Seen on |
|---|---|---|---|
| 2% | 3.00 gal | 20 gal | Older or lighter-duty SCR systems |
| 2.5% | 3.75 gal | 25 gal | A reasonable fleet average |
| 3% | 4.50 gal | 30 gal | Common heavy-duty estimate |
DEF freezes at about 12 °F (−11 °C). That is normal and does no harm — vehicle systems are designed to thaw it — but it matters for how you store containers. DEF is also a precise 32.5% urea solution: it must never be diluted with water or topped up with anything else, and it has a shelf life that shortens in heat.
Coolant concentration and freeze protection
Typical ethylene glycol figures. Your coolant's own chart governs — propylene glycol and the various extended-life chemistries all behave differently, and the numbers below are for orientation, not a specification.
| Concentrate | Freeze protection | Use |
|---|---|---|
| 40% | about −12 °F (−24 °C) | Mild climates; near the minimum for corrosion protection |
| 50% | about −34 °F (−37 °C) | The standard mix nearly everywhere |
| 60% | about −62 °F (−52 °C) | Severe cold — and the maximum many engine makers allow |
| 100% | about +9 °F (−13 °C) | Never. Neat concentrate protects worse than any mix |
More is not better, and past a point it is worse. Water carries heat away better than glycol does, so raising the concentration reduces cooling capacity and can cause overheating — which is why many engine manufacturers cap the mix at 60% even in severe cold. Protection also stops improving somewhere around 60–70% and then reverses, until neat concentrate freezes at a higher temperature than a 50/50 mix. The calculator will let you enter any percentage; check the engine maker's limit before you act on one above 60%. Note too that many coolants are sold pre-mixed at 50/50 — treating those as concentrate and adding water again gives you roughly 25%, which is not enough for either freeze or corrosion protection.
Two-stroke mix ratios
| Ratio | Oil per US gallon | Oil per litre | Typically |
|---|---|---|---|
| 20:1 | 6.40 oz | 50.0 mL | Older equipment and some vintage engines |
| 25:1 | 5.12 oz | 40.0 mL | Older two-strokes |
| 32:1 | 4.00 oz | 31.2 mL | Common on older saws and trimmers |
| 40:1 | 3.20 oz | 25.0 mL | Many modern handheld tools |
| 50:1 | 2.56 oz | 20.0 mL | Most current small engines |
The ratio comes from the equipment's manual, not from the oil bottle and not from what the last machine used. Too little oil seizes an engine; too much fouls plugs, carbons the exhaust and can also damage it. Mixed fuel does not keep — ethanol-blended petrol separates and degrades in weeks, so mix what you will use.
Where this goes wrong
- Using a brochure MPG. Use your own recent figure. Load, terrain, weather, idling and speed move real economy more than any spec does.
- Forgetting the return leg. There is a round-trip checkbox; an empty return still burns fuel.
- Adding water to pre-mixed coolant. Read the container. Half of all coolant sold is already diluted.
- Chasing freeze protection past 60%. It costs cooling capacity, and beyond the eutectic it costs freeze protection too.
- Mixing coolant chemistries. Incompatible types can gel and destroy a cooling system. Flush rather than top up with something different.
- Treating a DEF percentage as exact. It varies with duty cycle. Plan tank fills with margin.
- Guessing a two-stroke ratio. One wrong tank can end an engine.
Where these numbers come from
Fuel cost is arithmetic. DEF dose rate and coolant concentration are chemistry with a specification behind them, and getting either wrong is an expensive repair rather than a rounding error.
- ASTM D3306 — The specification for glycol-base engine coolant, including why concentration and water quality both matter.
- fueleconomy.gov (DOE/EPA) — Official fuel-economy data and the methodology behind published mpg figures.
- DOE Alternative Fuels Data Center — Fuel price series and energy-content comparisons across diesel and the alternatives.
Why not just run 100% coolant for maximum protection?
Pure antifreeze has worse heat transfer and freeze protection than a proper water/coolant mix — most manufacturers cap the recommended concentration around 60–70% coolant even in extreme cold.
Does DEF consumption really scale directly with diesel usage?
Roughly, yes, for planning purposes — actual DEF consumption depends on engine load, aftertreatment system, and duty cycle, so treat the percentage as an estimate and track actual usage over time for a more precise number.
Why does 2-stroke mix ratio matter so much?
Too little oil and the engine runs without enough lubrication, risking piston and bearing damage; too much oil fouls spark plugs and increases smoke and carbon buildup. Always match the ratio to your specific engine manufacturer's spec, not just what's common.
Maintenance intervals guide
DEF dosing rate and coolant mix are two of the fluids this page covers. For typical oil, coolant, DEF, and fuel filter service intervals by duty cycle, see the diesel engine maintenance intervals & fluids guide.
Estimates only. Confirm DEF dosing behavior and coolant mix limits against your specific engine or vehicle manufacturer's documentation.