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How to Calculate MPG
Miles per gallon (MPG) is distance driven divided by fuel consumed:
\[\text{MPG} = \frac{\text{Miles driven}}{\text{Gallons used}}\]Drive 350 miles on 12.5 gallons and your car gets 28 MPG. That single number tells you how efficiently your vehicle converts fuel into distance, and tracking it over time reveals whether your driving habits, tire pressure, or engine condition are changing.
The tank-fill method
The most accurate way to measure real-world MPG uses two consecutive fill-ups at the gas pump.
Fill your tank completely and note the odometer reading (or reset the trip odometer to zero). Drive normally until the tank is low enough to refill. Fill the tank completely again and record two numbers: the miles driven since the last fill-up and the gallons pumped to refill the tank.
Divide the miles by the gallons. That is your MPG for that tank.
The key to accuracy is filling to the same level both times. The automatic shutoff on the pump provides a consistent stopping point. Do not try to top off beyond the first click, as the extra fuel amount varies and introduces error.
Worked example
Suppose you reset your trip odometer at the pump, then drive for a week. At the next fill-up, the trip odometer reads 324.7 miles and the pump dispenses 11.2 gallons.
\[\text{MPG} = \frac{324.7}{11.2} = \textbf{29.0}\]Your car averaged 29.0 miles per gallon over that tank. If your car’s EPA rating is 32 MPG highway and you did mostly city driving, 29 MPG is a reasonable real-world result. If the number is significantly lower than expected, it may point to underinflated tires, aggressive driving, or a maintenance issue.
Tracking MPG over time
A single tank measurement is useful, but tracking MPG over many fill-ups gives a clearer picture. Seasonal changes, different routes, and driving style all affect fuel economy. Recording the miles and gallons at every fill-up takes 10 seconds and builds a useful data set.
Here is what a simple tracking log might look like:
| Fill-up date | Miles driven | Gallons | MPG |
|---|---|---|---|
| Jan 5 | 310 | 10.8 | 28.7 |
| Jan 19 | 287 | 10.1 | 28.4 |
| Feb 2 | 342 | 11.4 | 30.0 |
| Feb 15 | 295 | 10.6 | 27.8 |
The average across these four fill-ups is (28.7 + 28.4 + 30.0 + 27.8) / 4 = 28.7 MPG. A sudden drop in one reading is not necessarily a problem. But if MPG trends downward over several fill-ups, something has changed.
MPG vs. gallons per 100 miles
MPG has an unintuitive property: the fuel savings from improving MPG are not linear. Going from 10 MPG to 15 MPG saves far more fuel than going from 30 MPG to 35 MPG, even though both are a 5 MPG improvement.
This becomes obvious when you convert to gallons per 100 miles, which measures consumption directly.
| MPG | Gallons per 100 miles |
|---|---|
| 10 | 10.0 |
| 15 | 6.67 |
| 20 | 5.0 |
| 25 | 4.0 |
| 30 | 3.33 |
| 35 | 2.86 |
Improving from 10 to 15 MPG saves 3.33 gallons per 100 miles. Improving from 30 to 35 MPG saves only 0.47 gallons per 100 miles. This is why replacing a gas-guzzling truck with a moderately efficient one saves more fuel nationally than replacing efficient sedans with hybrids.
The formula to convert is:
\[\text{Gallons per 100 miles} = \frac{100}{\text{MPG}}\]Many countries use liters per 100 kilometers instead. To convert MPG to L/100km, divide 235.215 by the MPG value. A car rated at 30 MPG uses approximately 7.84 L/100km.
EPA ratings vs. real-world MPG
The numbers on the window sticker of a new car come from EPA laboratory tests conducted under controlled conditions. Real-world MPG almost always differs. The EPA itself notes that individual results will vary.
Several factors explain the gap. EPA tests use standardized driving cycles that may not reflect your actual commute. Temperature affects fuel economy, with cold weather reducing MPG by 10% to 20% for short trips because the engine runs less efficiently when cold and takes longer to warm up. Elevation changes, cargo weight, and the use of air conditioning all play a role as well.
As a rough guideline, most drivers get approximately 10% to 15% fewer miles per gallon than the EPA combined rating. If the sticker says 32 MPG combined, expect 27 to 29 MPG in typical mixed driving. Highway-only driving comes closer to the EPA highway number, while city-only driving may fall below the EPA city number.
Factors that affect fuel economy
Tire pressure is the easiest variable to control. Underinflated tires increase rolling resistance, which forces the engine to work harder. The Department of Energy estimates that properly inflated tires improve fuel economy by up to 3%. Check pressure monthly and inflate to the value listed on the driver’s door sticker, not the maximum printed on the tire sidewall.
Speed has a large effect. Most cars reach peak efficiency between 35 and 55 mph. Above 55 mph, aerodynamic drag increases rapidly. Driving at 70 mph instead of 55 mph can reduce fuel economy by 15% to 25%, depending on the vehicle’s shape and weight. The fuel cost calculator can show how speed-related MPG changes affect the cost of a trip.
Aggressive driving wastes fuel. Rapid acceleration and hard braking can lower MPG by 15% to 30% in city driving and 10% to 15% on the highway. Smooth, gradual acceleration and coasting to a stop keep the engine operating in a more efficient range.
Excess weight also matters, though less than people assume. Every 100 pounds of extra cargo reduces MPG by roughly 1% to 2%. Removing heavy items from the trunk helps, but the effect is modest compared to tire pressure and driving style.
Roof racks and cargo boxes increase aerodynamic drag significantly. Even an empty roof rack can reduce highway fuel economy by 2% to 5%. A large rooftop cargo box can reduce it by 10% to 25% at highway speeds. Removing roof accessories when not in use is one of the simplest ways to recover lost MPG.
When MPG drops unexpectedly
A sustained drop in MPG that cannot be explained by weather, route changes, or driving habits may indicate a mechanical issue. Dirty air filters, failing oxygen sensors, worn spark plugs, and misaligned wheels all reduce fuel efficiency. A check-engine light often accompanies these problems. If your tracked MPG drops by more than 10% from its historical average and stays there, a trip to a mechanic is worth the cost. The fuel savings from fixing the underlying issue typically pay for the repair within a few months of driving.