Fuel Economy Calculators
Work out MPG, fuel cost per trip or month, EV range and efficiency, 2-stroke mix ratio, and how far you can go before the tank runs dry.
How Fuel Economy Calculators Compare MPG and L/100km
Fuel economy (MPG, km/L) tells you distance per unit of fuel – higher is better. Fuel consumption (L/100km) is the inverse: how much fuel it takes to cover a fixed distance – lower is better.
Both describe the same car, just phrased in opposite directions, which is why converting between them trips people up. A more efficient car sees its MPG go up and its L/100km go down at the same time – they’re just two ways of reporting one improvement.
Cost Per Mile vs. the IRS Mileage Rate
The IRS standard mileage rate isn’t your actual cost per mile – it’s a flat reimbursement figure meant to average out fuel, depreciation, insurance, and maintenance across all vehicles. For 2026 it started at 72.5 cents per business mile, then the IRS raised it mid-year to 76 cents from July 1 onward, citing the run-up in fuel prices.
Your real cost per mile depends on the actual car you drive, so a fuel-efficient compact can easily run several cents below the IRS rate while a large SUV runs several cents above it. Comparing the two numbers side by side is the only way to know whether a per-mile reimbursement is actually covering what your vehicle costs to operate.
Turning Fuel Into Trip and Monthly Costs
A single trip’s fuel cost is just distance divided by fuel economy, multiplied by the price per gallon – simple until you’re trying to budget for a whole month of mixed driving.
Monthly fuel cost estimates work the same math over your typical monthly mileage instead of one trip, which is more useful for budgeting than a single tank-fill number, since it smooths out the difference between a light week and a road-trip week.
How Far Your Tank Will Actually Take You
Remaining range depends on two things: how much fuel is actually left in the tank, and your real-world MPG rather than the EPA sticker number. Tank shape matters more than people expect – a tank that’s shaped like a cone or has a saddle around the driveshaft doesn’t empty at a constant rate per inch of fuel level, which is why a dedicated tank-volume calculation beats eyeballing the gauge.
Combining an accurate tank volume with your actual measured MPG gives a range estimate that accounts for a car that runs richer or leaner than its window sticker suggests.
What Idling Actually Costs You
A typical passenger car burns about 0.2 to 0.5 gallons of fuel per hour at idle, according to Department of Energy and EPA guidance – roughly double that with the air conditioning running hard.
The DOE’s own rule of thumb is that restarting a modern fuel-injected engine burns about as much fuel as 10 seconds of idling, so any stop longer than that already costs more to idle through than to shut off and restart.
Fuel Weight Isn’t the Same Across Fuel Types
Weight = volume × density, and density varies by fuel: gasoline runs about 6.1 to 6.3 lb/gallon under ASTM D4814, diesel #2 is heavier at roughly 7.1 lb/gallon under ASTM D975, and propane is much lighter at about 4.2 lb/gallon.
That gap matters for anything weight-limited – a truck loaded to its GVWR with diesel is carrying meaningfully more fuel weight than the same tank filled with gasoline, which is why fuel type, not just fuel volume, belongs in any payload calculation.
EV Efficiency in Miles per kWh
Miles per kWh is the EV equivalent of MPG – distance covered per unit of energy, calculated as miles driven divided by kWh used.
Most modern EVs average 3 to 4 mi/kWh, with efficient compact models reaching 5 and larger trucks or SUVs sometimes dropping to 2 to 2.5. Multiplying mi/kWh by 33.7 converts it to MPGe for a rough comparison against a gas vehicle.
Getting the Oil-to-Gas Ratio Right for Small Engines
Most handheld 2-stroke equipment made before 2003 runs a 32:1 mix; most made after 2002 uses 40:1 or 50:1, and outboard motors typically run 50:1.
At 40:1, that’s 3.2 fl oz of oil per US gallon of gas; at 50:1, it’s 2.6 fl oz. Too little oil risks engine damage from poor lubrication, too much causes excess smoke and carbon buildup, and the ratio on the equipment label always overrides any general rule.
What a Cetane Index Number Actually Tells You
Cetane rating measures how readily diesel ignites under compression rather than from a spark – a higher number means shorter ignition delay and less of the knocking sound associated with a cold or low-quality diesel.
A true cetane number comes from a standardized engine test, which most fuel labs can’t run on every batch, so ASTM D4737 and the older D976 calculate an index instead, working backward from a fuel sample’s density and distillation temperatures. US diesel is specified to a minimum cetane number of 40 under ASTM D975.
From Crude Oil Price to What You Pay at the Pump
Crude oil is the single largest factor in the retail gasoline price, and the EIA reports it has averaged slightly more than 50% of the pump price over the past decade – the rest splits between refining margins, distribution and marketing, and taxes.
That share moves with the oil market itself: EIA’s 2026 outlook has crude oil’s contribution to the pump price falling below 45% on rising global supply, even as short-term spikes from events like Middle East supply disruptions can push crude’s share back up within weeks.
Estimating Annual Mileage Before It Costs You
Annual mileage isn’t just a curiosity – insurers set usage-based discount tiers around it, and warranty and lease mileage caps are built around an assumed yearly average.
Projecting from a normal week’s commute, errands, and the occasional road trip gives a more realistic annual figure than guessing, and catches drivers who are quietly on track to blow past a lease’s mileage allowance months before the bill for it arrives.
Frequently Asked Questions
How do I know which oil mix ratio my engine needs?
Check the equipment label or owner’s manual first. As a rule of thumb, handheld equipment built before 2003 typically uses 32:1, equipment built after 2002 usually uses 40:1 or 50:1, and most outboard motors use 50:1.
What’s a good miles-per-kWh number for an EV?
Most modern EVs land between 3 and 4 mi/kWh. Anything above 4 is efficient; larger EVs or trucks running closer to 2 to 2.5 mi/kWh is normal for the vehicle class, not a sign of a problem.
Why do MPG and L/100km move in opposite directions for the same improvement?
MPG measures distance per fuel – a bigger number is better. L/100km measures fuel per distance – a smaller number is better. A more efficient car sees MPG go up and L/100km go down at the same time; they’re just inverse ways of describing it.
How does the IRS mileage rate differ from what Fuel Economy Calculators show as cost per mile?
The IRS rate – 76 cents per business mile as of July 2026 – is a flat, one-size-fits-all reimbursement figure covering fuel, depreciation, insurance, and upkeep together. Your actual cost per mile is specific to your vehicle’s real fuel economy and local fuel price, and can land well above or below the IRS figure depending on what you drive.
How much fuel does idling actually waste?
A typical car burns roughly 0.2 to 0.5 gallons an hour at idle per DOE and EPA guidance, and running the AC hard can push that close to double. Ten minutes of daily idling adds up to well over 20 gallons a year for the average driver – burned without covering a single mile.
Why does a gallon of diesel weigh more than a gallon of gasoline?
Diesel is a denser fuel – about 7.1 lb/gallon under ASTM D975, versus roughly 6.1 to 6.3 lb/gallon for gasoline under ASTM D4814. That extra density is part of why diesel carries more energy per gallon and typically returns better fuel economy than gasoline in a comparable engine.
How much of the price at the pump is actually crude oil?
Over the past decade, crude oil has made up slightly more than half of the average US retail gasoline price, with the rest split between refining, distribution and marketing, and taxes. The EIA’s 2026-2027 outlook has that share falling below 45% on rising global oil supply, though short-term supply shocks can push it back up quickly.
What does a cetane index number actually tell me about diesel quality?
It estimates how easily the fuel ignites under compression – higher means a shorter ignition delay and less knock, especially in cold starts. US diesel must meet a minimum cetane number of 40 under ASTM D975, and most calculators use the D4737 or D976 index methods since they only need a fuel sample’s density and distillation data, not a full engine test.