Gallons per 100 Miles to MPG Converter

Convert fuel consumption from gallons per 100 miles to miles per gallon (MPG) and back. Supports both US gallons and UK/Imperial MPG, with optional fuel cost and CO₂ impact estimates.

Gallons per 100 miles ➜ MPG

Example: 4 gallons/100 miles ≈ 25 MPG (US).

Result:

25.00 MPG (US)

MPG ➜ Gallons per 100 miles

Example: 30 MPG ≈ 3.33 gallons/100 miles (US).

Result:

3.33 gallons/100 miles (US)

Compare two cars & estimate cost

~8.89 kg CO₂/US gal for gasoline (EPA).

How to convert gallons per 100 miles to MPG

Gallons per 100 miles (gal/100 mi) tells you how many gallons of fuel your car uses to drive 100 miles. Miles per gallon (MPG) tells you how far you can drive on one gallon. They are mathematical inverses.

Formulas

From gallons/100 miles to MPG:

\[ \text{MPG} = \frac{100}{\text{gallons per 100 miles}} \]

From MPG to gallons/100 miles:

\[ \text{gallons per 100 miles} = \frac{100}{\text{MPG}} \]

Example conversions

  • 4 gal/100 mi → \(100 / 4 = 25\) MPG
  • 5 gal/100 mi → \(100 / 5 = 20\) MPG
  • 30 MPG → \(100 / 30 ≈ 3.33\) gal/100 mi
  • 20 MPG → \(100 / 20 = 5\) gal/100 mi

US MPG vs UK (Imperial) MPG

This converter supports both US and UK/Imperial units:

  • US gallon: 1 US gal ≈ 3.785 L
  • UK gallon: 1 UK gal ≈ 4.546 L (about 20% larger)

Because the UK gallon is larger, the same car will show a higher MPG number in UK MPG than in US MPG. The underlying fuel use (liters per 100 km, or energy) is the same; only the unit changes.

Why gallons per 100 miles can be clearer than MPG

Many experts prefer fuel used per distance (like gal/100 mi or L/100 km) because it is linear in fuel consumption. Each change in gal/100 mi corresponds directly to a change in fuel used.

For example, suppose you drive 12,000 miles per year:

  • Car A: 5 gal/100 mi (20 MPG)
  • Car B: 4 gal/100 mi (25 MPG)

Over 12,000 miles:

  • Car A uses \( 5 × 120 = 600 \) gallons
  • Car B uses \( 4 × 120 = 480 \) gallons
  • Savings: 120 gallons per year

The difference between 20 and 25 MPG might not look huge, but in gal/100 mi you can immediately see that Car B uses 1 gallon less per 100 miles, which is easy to translate into annual savings.

Typical gallons per 100 miles values

Approximate ranges for passenger cars using US gallons:

  • 6–8 gal/100 mi (12–17 MPG): large SUVs, trucks, or older vehicles
  • 4–6 gal/100 mi (17–25 MPG): many conventional gasoline cars
  • 3–4 gal/100 mi (25–33 MPG): efficient compact cars, some hybrids
  • < 3 gal/100 mi (> 33 MPG): very efficient hybrids or small cars

FAQ

How do I manually convert gallons per 100 miles to MPG?

Divide 100 by the gallons per 100 miles value. For example, if your car uses 3.5 gallons per 100 miles:

\[ \text{MPG} = \frac{100}{3.5} ≈ 28.57 \]

How do I convert MPG to gallons per 100 miles?

Again, divide 100 by the MPG value. For example, if your car gets 32 MPG:

\[ \text{gal/100 mi} = \frac{100}{32} ≈ 3.13 \]

Does this converter change between US and UK gallons?

Yes. Use the Unit system dropdown to choose US or UK/Imperial. The math (100 divided by the value) is the same, but the meaning of “gallon” and “MPG” changes with the system you select.

How accurate are the cost and CO₂ estimates?

They are first-order estimates based on:

  • Your entered annual miles and fuel price
  • A default CO₂ factor of 8.89 kg per US gallon of gasoline (EPA average)

Real-world results will vary with driving style, fuel type, and conditions, but the comparison between two cars is usually quite informative.