Cernarus

Convert Miles per Gallon to Gallons per 100 Miles - Fuel Economy Converter

This converter turns vehicle fuel economy expressed in miles per gallon (mpg) into the amount of fuel a vehicle uses to travel 100 miles (gallons per 100 miles). Converting to gallons per 100 miles gives an intuitive measure of fuel consumption where lower numbers indicate better efficiency.

By default this tool assumes mpg is measured in US gallons. If your mpg value uses Imperial gallons (common in the UK), use the guidance below to adjust for the different gallon sizes. The calculation is exact for the mathematical relationship; real-world accuracy depends on odometer and fuel measurement precision.

Updated Nov 29, 2025

Interactive converter unavailable for this calculator.

We could not resolve compatible units for this experience. Please verify the slug follows the pattern `from-unit-to-unit-converter`.

Methodology

The relationship between mpg and gallons per 100 miles is driven by the reciprocal: gallons_per_100_miles = 100 / (miles_per_gallon). This yields fuel used over a fixed 100-mile distance, which is convenient for comparison across vehicles and test cycles.

When the source mpg is reported using Imperial gallons rather than US gallons, convert the mpg values between gallon definitions before applying the reciprocal. Use the physical definitions: 1 Imperial gallon = 4.54609 liters and 1 US gallon = 3.785411784 liters, so 1 Imperial gallon = 1.20095 US gallons.

Round results appropriately for display and for the precision of your inputs. For typical consumer-facing displays, two decimal places is common; for engineering or fleet reporting, preserve additional significant figures as required.

Worked examples

Example 1: 25.0 mpg (US) → gallons per 100 miles = 100 / 25.0 = 4.00 gal/100 mi.

Example 2: 40.0 mpg (Imperial) → convert to US mpg: 40.0 * 0.832674 = 33.31 mpg (US); then gallons per 100 miles = 100 / 33.31 = 3.00 gal/100 mi (rounded).

Key takeaways

Use gallons per 100 miles for a fixed-distance view of fuel consumption. Calculate it by taking 100 divided by the mpg (after ensuring mpg is expressed using the intended gallon definition).

Refer to NIST and DOE/EPA guidance when using these numbers for regulation, lab reports, or precise engineering decisions.

Expert Q&A

Why convert mpg to gallons per 100 miles?

Gallons per 100 miles expresses fuel use on a fixed-distance basis. It is useful for comparing consumption directly (lower is better) and is analogous to liters per 100 km commonly used in regions that report metric fuel economy.

Do I need to worry about US vs Imperial gallons?

Yes. An mpg reported with Imperial gallons is not numerically the same as mpg with US gallons. Convert Imperial mpg to US-equivalent mpg before applying the reciprocal using the factor 1 Imperial gallon = 1.20095 US gallons.

How precise is this conversion for real-world fuel economy?

The mathematical conversion is exact, but real-world accuracy depends on odometer accuracy, pump meter calibration, and how fuel fill events are measured. For regulatory or laboratory reporting follow measurement standards from agencies such as NIST and test procedures published by DOT/EPA/DOE.

How should I round the result?

For everyday use, round to two decimal places (e.g., 3.45 gal/100 mi). For engineering analyses or regulatory submissions, keep the precision required by your protocol and report the measurement uncertainty where relevant.

Can I use this to compare to liters per 100 km?

Yes. Convert mpg → gallons per 100 miles with this tool, then convert gallons to liters and miles to kilometers if you prefer liters per 100 km. Use 1 US gallon = 3.785411784 liters and 1 mile = 1.609344 kilometers for exact SI-consistent conversions.

Where do official test values (EPA/DOE) come from?

Official fuel-economy labels are derived from standardized laboratory tests and correction factors defined by agencies such as the U.S. EPA and DOE. Those protocols control test cycles, environmental corrections, and measurement procedures.

Sources & citations