MPG Calculator

Calculate Your Real-World Fuel Economy

Your car's EPA rating is measured under controlled conditions that rarely match real driving. The only way to know your actual fuel economy is to calculate it yourself from a fill-up — which takes two minutes and gives you a number you can actually use for budgeting, comparing vehicles, and tracking whether something is affecting your engine performance.

How to Use This Calculator

Enter the miles driven since your last fill-up (from your trip odometer), the gallons used (from your gas pump receipt), and the fuel price per gallon. Select a comparison vehicle to see how your fuel economy stacks up and what the annual cost difference is. Hit Calculate MPG to see your efficiency in MPG, km/L, and L/100km.

How to Get an Accurate MPG Reading

For the most accurate measurement:

1. Fill your tank completely to the same level (until the pump clicks off automatically)
2. Reset your trip odometer immediately after filling
3. Drive normally until your next fill-up
4. Fill to the same level and note the gallons pumped and miles on the trip odometer
5. Enter those two numbers here

A single tank calculation can be skewed by partial fills, varying driving conditions, or ethanol blend differences. Averaging 3–5 consecutive tanks gives a more reliable baseline.

How the Math Works

Note the inverse relationship: higher MPG = lower L/100km. Both measure the same thing — L/100km is the dominant metric in Canada, Australia, and Europe; MPG is standard in the US and UK.

Real-World Example

Using the defaults — 350 miles driven, 12.5 gallons used, $3.50/gal:

• MPG: 350 ÷ 12.5 = 28.0 MPG
• km/L: 28 × 0.4251 = 11.9 km/L
• L/100km: 235.21 ÷ 28 = 8.4 L/100km
• Cost per mile: $3.50 ÷ 28 = $0.125/mile
• Annual fuel cost (12,000 mi): $1,500

Compare to a 20 MPG truck at the same fuel price: $0.175/mile → $2,100/year. The difference of $600/year compounds to $6,000 over a typical 10-year ownership period — a real factor in total cost of ownership when choosing a vehicle.

What Affects Your Real-World MPG

• Speed — aerodynamic drag increases with the square of speed. Driving 75 mph vs 60 mph can reduce fuel economy by 15–20%.
• City vs. highway — stop-and-go city driving wastes energy in braking and idling. Highway driving is typically 20–35% more efficient for most vehicles.
• Temperature — cold weather thickens engine oil, increases rolling resistance from tires, and forces longer engine warm-ups. Fuel economy drops 10–15% in freezing temperatures.
• Tire pressure — underinflated tires increase rolling resistance. Every 1 PSI below optimal reduces fuel economy by ~0.2%.
• Load and drag — roof racks, cargo boxes, and heavy loads all increase fuel consumption. Remove roof racks when not in use — even empty ones add drag.
• Air conditioning — AC can reduce fuel economy by 5–25% depending on driving conditions. At highway speeds, the aerodynamic penalty of open windows may outweigh AC use.

Frequently Asked Questions