QuickUse Calculator

Gas Mileage Calculator: MPG, Cost & EV Compare

Calculate fuel economy, cost per mile, and yearly fuel cost. Compare two vehicles, hybrid vs gas, and EV cost equivalent — with mpg to km/L conversions.

Portrait of Ryan Clarke, software engineer and founder of QuickUse CalculatorBy Software Engineer & Site Founder
Reviewed by on .Sources verified against 8 official references.
Based on 6 official sources↓
Mode
Vehicle
mpg
mi
Prices
$
One-off trip (optional)
mi

Your fuel economy

30.0 mpg

Equivalent: 12.8 km/L / 7.8 L/100km

Cost

Cost per mile$0.1167/mi
Monthly cost$117
Annual cost$1,400

Educational notes

  • • 30 mpg → mid-size sedan
  • • EPA window-sticker numbers are tested under controlled conditions. Real-world combined economy is typically 10-15% lower for ICE vehicles, 20-30% lower for full hybrids in cold weather. Adjust accordingly when budgeting.
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Gas mileage looks like a simple division — distance ÷ fuel — and it is, until you start comparing across markets, units, and fuels. The US still measures in miles per gallon and the rest of the world in km/L or L/100km. Hybrids quote MPG-equivalent (MPGe) using a 33.7 kWh/gal energy-equivalent factor most drivers will never internalise. Real-world fuel economy is consistently 10-15% worse than EPA window-sticker numbers because the EPA test cycle does not include hard acceleration, A/C use, hills, or cold-start fuel enrichment. This calculator handles the unit conversion side, then layers on the cost-per-mile math, EV comparisons, and a Brazil-specific killer feature: the calibrable 70% rule for ethanol vs gasoline.

On the US side the headline value is "what does my year of driving cost". $3.50/gal × 12,000 mi ÷ 30 mpg = $1,400/yr — sounds modest until you put it next to a 20-mpg pickup at $2,100/yr or a 50-mpg Prius at $840/yr. Stretch the comparison over 10 years and the gap is $9,000-13,000 in fuel alone. The vehicle-comparison mode surfaces this directly so you can see whether the hybrid premium pays back inside the ownership period (Toyota Prius at $4-6k more than equivalent Camry typically pays back in 8-10 years for 12k-mile drivers, faster for 18k-mile drivers).

On the Brazilian side the calculator does something most local competitors don't: it lets you calibrate the 70% rule. The folkloric "if ethanol is below 70% of gasoline price, ethanol wins" comes from the 5,500 / 8,000 kcal/L heating-value ratio (≈ 68.75%, rounded). But real-world flex engines vary 65-75% by compression ratio, and since August 2025 the gasoline at the pump is E30 (30% anhydrous ethanol, up from E27), which narrows the differential. The calculator surfaces all three: the price ratio, your custom factor (or the default 70%), and — best of all — accepts your measured km/L on each fuel for a personalised comparison that bypasses the approximation entirely.

The math behind fuel economy

Unit conversions. 1 mpg = 0.4251 km/L (because 1 mile = 1.60934 km and 1 US gallon = 3.78541 L; the ratio is 1.60934 / 3.78541). The inverse is 1 km/L = 2.3521 mpg. L/100km is the reciprocal of km/L scaled by 100: 12 km/L = 100/12 = 8.33 L/100km. Lower L/100km = better; higher km/L or mpg = better. The calculator normalises whichever unit you pick into all three so you can compare cars sold in different markets without doing the math yourself.

Cost per distance. US: $/mile = price_per_gallon ÷ mpg. BR: R$/km = price_per_liter ÷ km_per_liter. Multiply by annual mileage to get yearly cost. The calculator uses Decimal arithmetic for cent-level precision because compounded errors over 100k miles get noticeable.

EV cost equivalence. EVs report kWh per 100 miles (or 100 km outside the US). At $0.15/kWh × 30 kWh/100mi = $0.045/mile, vs a 30-mpg ICE at $3.50/gal = $0.117/mile. EV is ~62% cheaper per mile under those assumptions. The calculator handles this in vehicle-comparison mode — set fuel_type=ev and provide kWh/100mi as the "fuel economy value".

70% rule (BR) — origin and limits. Ethanol fuel has a heating value of ≈ 5,500 kcal/L; gasoline ≈ 8,000 kcal/L. The energy ratio is 5500/8000 = 0.6875, rounded to 70% in popular usage. So if 1 L of ethanol delivers 30% less energy than 1 L of gasoline, you need ~30% more ethanol to drive the same distance — which means ethanol only wins on cost when its price is ≤ 70% of gasoline price. Reality: modern flex engines with 12:1+ compression ratios can extract more energy from ethanol (effective ratio up to 75%), and E30 gasoline already contains 30% ethanol so the differential is smaller than it was in 2010. The calibration slider (65-75%) accounts for engine variation; the "measured km/L" inputs bypass the rule entirely and use your actual data.

GNV (Brazil only). Compressed natural gas runs ~30-40% cheaper than gasoline at the pump (R$ 4/m³ vs R$ 6/L, with typical km/m³ of 12-15 on a 1.6 flex). Installation kit costs R$ 4-7k. Payback works out to 8-15 months at 1,500+ km/month — but below that threshold the trunk-space penalty and 5-yearly cylinder revalidation make it uneconomical. The viability calc combines all three signals (monthly mileage threshold, payback ceiling, sweet-spot recommendation).

Adulteration warning (BR). ANP regularly autua posts selling gasoline with 40-70% ethanol (well above the legal 30%). Telltale signs: prices well below the regional average, no fiscal receipt, no major-brand flag. Since E30 already mixes 30% ethanol, detecting illegal "extra" ethanol by smell or pump performance is harder than it was. The calculator flags suspiciously low gasoline prices in the warnings section.

Fuel economy and cost-per-distance

mpg ↔ km/L: 1 mpg = 1.60934/3.78541 km/L = 0.4251 km/L 1 km/L = 2.3521 mpg L/100km = 100 / km/L (inverse) Cost per mile (US) = price_per_gallon / mpg Cost per km (BR) = price_per_liter / km_per_liter Annual cost = cost_per_distance × annual_mileage 70% rule (BR): ratio = price_ethanol / price_gasoline ethanol wins if ratio ≤ calibration_factor default 0.70, range 0.65-0.75

price
pump price per liter or gallon
mpg / km/L
distance per unit of fuel
calibration_factor
engine-specific ethanol/gasoline ratio (0.65-0.75)
kWh/100mi
EV energy consumption (Tesla Model 3 ≈ 27-30)

Practical examples

US — F-150 (20 mpg) vs Camry (32 mpg), 15k mi/yr, $3.50/gal

Setup: Two-driver household choosing between a Ford F-150 (20 mpg combined) and a Toyota Camry (32 mpg combined). Both at 15,000 miles/year. Gas at $3.50/gallon (national average April 2026).

**F-150:** $3.50 / 20 = $0.175/mile × 15,000 = **$2,625/year**. **Camry:** $3.50 / 32 = $0.109/mile × 15,000 = **$1,640/year**. **Annual savings of Camry: $985**. Over 5 years: $4,922. Over 10 years: $9,844 in fuel alone — before considering Camry has 30%+ better resale value and lower maintenance.

Takeaway: A 12-mpg gap (60% better) translates to ~$10k over a decade just in fuel. If the F-150 buyer needs the truck capability 4 days a month and a sedan would suffice the rest of the time, renting a truck for those 4 days ($75 × 48/year = $3,600) is cheaper than owning the truck full-time. The math forces a hard look at why you chose the larger vehicle.

US — Hybrid (50 mpg) vs Sedan (30 mpg), 12k mi/yr

Setup: Toyota Prius at 50 mpg combined vs Camry at 30 mpg, both at 12,000 miles/year and $3.50/gal. Prius costs ~$4,000 more new than equivalent Camry trim.

**Prius:** $3.50 / 50 = $0.07/mi × 12,000 = **$840/year**. **Camry:** $3.50 / 30 × 12,000 = **$1,400/year**. **Savings: $560/year**. Payback on $4,000 premium: 7.1 years for fuel alone. Add federal/state hybrid incentives (varies) and maintenance (Prius brakes last 100k+ miles vs 50-60k for ICE) and payback often shrinks to 4-5 years.

Takeaway: Hybrid premium pays back inside ownership period for most buyers — but only if you actually drive 10k+ miles/year. Low-mileage urban drivers (5k mi/year) extend payback past 14 years, by which point battery replacement enters the equation. Match the powertrain to actual usage.

US — EV cost equivalent: Tesla Model 3 vs Camry

Setup: Tesla Model 3 at 30 kWh/100mi (real-world) vs Camry at 30 mpg, both at 12,000 miles/year. Electricity at $0.15/kWh (US national residential average). Gasoline at $3.50/gal.

**Tesla:** $0.15 × 30 / 100 = $0.045/mile × 12,000 = **$540/year**. **Camry:** $3.50 / 30 × 12,000 = **$1,400/year**. **Savings: $860/year**. At California peak rates ($0.30/kWh), EV cost rises to $1,080 — savings narrow to $320/year. With DC fast charging at $0.45/kWh, EV becomes more expensive than gas.

Takeaway: EV economics depend heavily on home-charging access and electricity tariff. The $860/year national-average savings disappears in California time-of-use peak hours. If you cannot install Level 2 home charging, run the calc with public-charging rates ($0.40-0.55/kWh) — the picture changes.

BR — Etanol R$ 4 vs Gasolina R$ 6, regra 70%

Setup: Onix flex 1.0 a 12 km/L (gasolina). Etanol R$ 4,00/L, gasolina R$ 6,00/L. Default factor 70%. 15.000 km/ano.

**Razão:** R$ 4 / R$ 6 = **66,7%** — abaixo do fator 70%. **Etanol vence.** Custo etanol/km estimado: R$ 4 / (12 × 0.70) = R$ 0,476/km. Custo gasolina/km: R$ 6 / 12 = R$ 0,50/km. **Diferença: R$ 0,024/km × 15.000 = R$ 360/ano.** Pequena mas constante. Se motor for mais eficiente com etanol (ratio real 0.75), economia sobe.

Takeaway: Differença anual modesta (R$ 360) sugere que vale escolher por outros critérios: disponibilidade no posto da rota, confiança em postos com flag conhecida (evita adulteração), preferência por menos partícula no escapamento (etanol queima mais limpo). Para resultado mais preciso, use a calc com seus consumos medidos (2 abastecimentos cada).

BR — Motorista app, GNV, 3.000 km/mês

Setup: Sedan flex 11 km/L gasolina. Roda 3.000 km/mês (motorista de app). Gasolina R$ 6/L, GNV R$ 4/m³, kit GNV R$ 5.500. Consumo GNV típico 14 km/m³.

**Custo gasolina mensal:** 3.000 / 11 × R$ 6 = **R$ 1.636/mês**. **Custo GNV mensal:** 3.000 / 14 × R$ 4 = **R$ 857/mês**. **Economia mensal: R$ 779**. **Payback do kit: R$ 5.500 / R$ 779 ≈ 7 meses**. Economia anual: **R$ 9.351**. Recomendação: instalar.

Takeaway: GNV é game-changer para uber/99/táxi com 2.500+ km/mês. Trade-offs: porta-malas reduzido (cilindro), revalidação INMETRO a cada 5 anos (~R$ 200), e perda de potência leve. Para uso pessoal abaixo de 1.500 km/mês, raramente compensa — calc mostra "não viável" automaticamente.

Common pitfalls in fuel economy decisions

  • Driving 5 miles to save $0.10/gallon eats the savings. A 10-mile round trip in a 25-mpg car costs ~$1.40 in gas. If you save $0.10/gal × 10 gallons = $1.00 in the fill-up, you net-lose $0.40 plus your time. Stick with the closest reasonable station unless you are passing the cheaper one anyway.
  • Tire pressure drives 3-5% MPG variation. Properly inflated tires (check the door-jamb sticker, not the sidewall) save ~10-15 cents/gallon equivalent. Under-inflation by 10 PSI loses 1-2 mpg in a typical sedan. Free, takes 5 minutes monthly.
  • Premium fuel in a regular-spec car is wasted money. Engines designed for 87 octane gain zero performance and zero MPG from 91-93 octane. Premium only helps in high-compression turbo engines that knock on regular. Check your owner's manual; it specifies "regular" or "premium required" — the latter is rare on non-luxury vehicles.
  • EPA sticker is best-case. The 30 mpg combined number on the window sticker comes from a specific test cycle. Real-world combined economy averages 10-15% worse for ICE, 20-30% worse for hybrids in cold weather (battery efficiency drops). Budget conservatively — assume 27 mpg if the sticker says 30.
  • Roof racks and cargo boxes hurt aerodynamics. A loaded roof box on a sedan loses 1-2 mpg sustained at highway speed. Take it off when not in use. The "I leave it on year-round" convenience easily costs $100/year in extra fuel.

Limits of this calculator

Commercial fleet operations: depreciation, fuel taxation (IFTA-style cross-state diesel tax), maintenance amortisation differ from personal vehicle math. Use a fleet TCO tool.

EV with home solar: if you generate your own electricity at near-zero marginal cost, set fuel_price_electricity ≈ $0.02 (transmission losses + system amortisation). The calc handles this; just adjust the input.

Plug-in hybrids (PHEVs): the calc does not separately model the gas-vs-electric split. Set the value to a blended MPGe combining electric-mode (e.g. 100 MPGe for first 30 mi) and gas-mode (45 mpg after battery depletes). For a 12k-mile/year driver doing 30 mi/day all-electric, treat as EV.

Diesel highway-vs-city differential: diesel engines are typically 25-30% more efficient on highway than city. The calc uses combined economy. If you do 80% highway, bump the input mpg/km/L by ~10% from sticker.

Brazilian "atypical" fuel scenarios: gasolina premium (E27 não-misturada) is sold at ~10% more than regular E30 with marginal MPG benefit. The calc treats it the same as regular gasoline; if you fuel premium, lower fuel_economy_value by 2-3% in the input to be conservative.

Frequently asked questions

Why is my real MPG lower than the EPA sticker?▾

EPA test cycle excludes hard acceleration, A/C use, hills, cold starts, and roof racks. Real-world combined economy averages 10-15% lower for ICE and 20-30% lower for hybrids in cold weather. The fueleconomy.gov website shows user-reported real-world numbers next to the sticker — usually closer to your experience.

What is MPGe and how does it compare to MPG?▾

MPGe (miles per gallon equivalent) is the EPA's metric for EVs and PHEVs. It uses 33.7 kWh as the "energy equivalent of a gallon of gasoline" so a Tesla Model 3 at 30 kWh/100mi rates ~112 MPGe. For cost comparison MPGe is misleading because electricity per kWh costs much less than the energy in a gallon of gas. Always compare cost per mile rather than MPG/MPGe directly.

How does E30 affect the 70% rule?▾

Since August 2025 Brazilian gasoline contains 30% anhydrous ethanol (was 27%). This narrows the real-world difference between ethanol and gasoline at the pump. The 70% rule is an approximation that gets slightly less accurate with each ethanol-content increase. Best practice: measure your own car's km/L on each fuel across two full tanks and compare directly.

When does GNV (CNG) make sense?▾

When you drive 1,500+ km/month consistently, gas prices are ≥ R$ 5.50/L, and GNV is available at ≤ R$ 4.50/m³ in your area. Below those thresholds the kit cost (R$ 4-7k) and trunk-space penalty outweigh savings. Common viable users: motoristas de app, taxistas, vendedores externos, frota leve.

Should I buy a hybrid or a regular ICE car?▾

Hybrid premium ($3-5k typical) pays back through fuel savings in 5-10 years for 12k+ mi/year drivers, faster for higher mileage. Below 8k mi/year the payback extends past battery-warranty period (8-10 years) and the math gets shaky. Hybrids also benefit from regenerative braking (less brake pad wear). For city-heavy driving, hybrid is almost always the right call; for highway-heavy, the gap narrows.

Does this calculator handle gallon UK vs gallon US?▾

Only US gallons (3.78541 L). UK gallons are 4.546 L (~20% larger). If you have a UK MPG figure, divide by 1.201 to convert to US MPG before entering. We do not auto-detect.

Why does the BR calc default to 12 km/L?▾

It is the median for Brazilian flex 1.0-1.4 cars in mixed urban driving (Onix, HB20, Polo, Argo). Pickups (Hilux, S-10) run 8-10 km/L. Sedans medium (Civic, Corolla) run 11-13 km/L. Hybrids (Corolla Cross Hybrid) run 18-20 km/L. Adjust to your actual measured value for accurate cost projection.

Why is fuel adulteration flagged when I enter a low gas price?▾

ANP autua dezenas de postos por mês com gasolina contendo 40-70% etanol (legal limit 30% via E30). Telltale signs: price > 10% below regional average, no fiscal receipt, suspicious flag. The calc flags inputs below R$ 5.50/L because the cheapest legitimate gas in 2026 BR market is around that floor — anything under is suspicious.

Sources & references

Cross-check every number in this calculator against the primary sources below.