MPG Fuel Efficiency Calculator — Calculate Miles Per Gallon and Litres Per 100km

Fuel Efficiency Units

How to Calculate Your Fuel Efficiency

1. Fill your tank completely. Reset your trip odometer to zero (or note the current odometer reading).
2. Drive normally until the tank is partially or fully depleted.
3. Fill the tank again completely. Note the exact amount of fuel added (in litres or gallons) and the distance driven (in miles or km).
4. Open the fuel efficiency calculator and enter the distance and fuel used.
5. The calculator returns MPG, L/100km, km/L, and estimated annual fuel cost based on your driving distance and local fuel price.

How to Convert Between Fuel Efficiency Units

Vehicle Fuel Efficiency Benchmarks

Why Your Real-World MPG Differs from Manufacturer Figures

Manufacturer fuel economy figures are measured under controlled test conditions — typically laboratory cycles that do not fully replicate real-world driving. Common reasons your actual fuel economy may differ:

Driving style

Aggressive acceleration and braking significantly reduce fuel efficiency. Smooth acceleration, gentle braking, and maintaining steady speeds on motorways maximise efficiency. Idling in traffic with the engine running is effectively 0 MPG — all fuel burned, no distance covered.

Speed

Aerodynamic drag increases with the square of speed. Driving at 80 mph instead of 70 mph increases air resistance by about 30% and reduces fuel efficiency noticeably. Most petrol and diesel engines are most efficient at moderate speeds (50–65 mph for many cars).

Air conditioning

Air conditioning can reduce fuel economy by 5–25% depending on conditions. At low speeds and high ambient temperatures, the AC compressor takes significant power from the engine. At high speeds, closing windows and using AC may actually be more efficient than the drag from open windows.

Tyre pressure

Under-inflated tyres increase rolling resistance. Maintaining correct tyre pressure (usually 32–36 PSI for most passenger cars — check the label inside the driver's door) can improve fuel economy by 0.5–3%.

Temperature

Cold weather reduces fuel efficiency for several reasons: engines take longer to reach optimal operating temperature, tyre pressure drops (reducing by ~1 PSI per 10°F drop in temperature), oil is more viscous when cold, and cabin heating draws energy. Hybrids and electric vehicles are particularly affected by cold weather.

Vehicle load

Extra weight reduces fuel efficiency — approximately 1–2% per 100 lbs of added weight. Roof boxes and roof racks add aerodynamic drag and can reduce highway efficiency by 10–25%. Remove roof racks when not in use.

Calculating Annual Fuel Cost

Annual fuel cost = (Annual miles / MPG) × fuel price per gallon

Example: 12,000 miles/year, 30 MPG, £1.50/litre (UK petrol), 4.546 litres/UK gallon:

1. UK gallons per year = 12,000 / 30 = 400 gallons
2. Litres per year = 400 × 4.546 = 1,818 litres
3. Annual cost = 1,818 × £1.50 = £2,727

The MPG calculator automates this calculation. Enter your annual mileage, efficiency, and local fuel price to get an accurate annual fuel cost estimate.

MPG vs MPGe for Electric and Hybrid Vehicles

The EPA (US Environmental Protection Agency) developed MPGe (Miles Per Gallon Equivalent) to allow comparison of electric and plug-in hybrid vehicles with conventional petrol vehicles. One gallon of petrol contains approximately 33.7 kWh of energy, so an EV using 33.7 kWh of electricity to travel a certain distance would have the same MPGe as a petrol car getting that many MPG.

A 2025 Tesla Model 3 Long Range has an EPA-rated efficiency of about 131 MPGe — equivalent to a petrol car getting 131 MPG, which is impossible for a combustion engine. This highlights how much more efficiently electric motors convert energy to motion compared to internal combustion engines (which waste 60–70% of fuel energy as heat).

Comparing Fuel Costs: Petrol vs. Diesel vs. Electric

When comparing running costs between fuel types, normalise to cost-per-mile:

• Petrol car (30 MPG, £1.50/L): Cost per mile = (1.50 × 4.546) / 30 = £0.23/mile
• Diesel car (45 MPG, £1.55/L): Cost per mile = (1.55 × 4.546) / 45 = £0.16/mile
• Electric car (4 miles/kWh, 28p/kWh home rate): Cost per mile = 0.28/4 = £0.07/mile

Public charging rates are higher than home rates, which narrows the gap. But for most drivers who can charge at home overnight, electric vehicles offer significant fuel cost savings over petrol or diesel.

Tips for Improving Fuel Efficiency

• Drive at steady speeds: Use cruise control on motorways where traffic allows.
• Anticipate stopping: Lift off the accelerator early and coast to traffic lights rather than braking hard at the last moment.
• Maintain correct tyre pressure: Check monthly; tyres lose pressure over time.
• Remove unnecessary weight: Clear boots of heavy items you don't need.
• Remove roof boxes when not in use: The aerodynamic drag penalty is significant at motorway speeds.
• Service your vehicle regularly: Blocked air filters, worn spark plugs, and old oil all reduce efficiency.
• Reduce short trips: Cold engines use significantly more fuel — short trips never allow the engine to reach optimal temperature.

Common Questions

What is the difference between US and UK gallons?

1 US gallon = 3.785 litres; 1 UK (Imperial) gallon = 4.546 litres. A UK gallon is 20% larger than a US gallon. This is why UK MPG figures appear higher than US MPG figures for the same vehicle — the UK gallon is bigger. To convert US MPG to UK MPG: multiply by 1.201. When comparing fuel efficiency figures, always check whether MPG is quoted in US or UK gallons.

How accurate are fuel efficiency calculations?

Tank-to-tank calculations are accurate if you fill the tank to the same level each time (always use the automatic click-off as the stopping point, not a round number). Avoid filling to the top after the auto cut-off — this adds inconsistency. For a precise measurement over one fill-up, three data points are ideal: fill up, drive a significant distance (200+ miles), fill up again to measure.

What is a good MPG for a petrol car?

This depends on the vehicle class. For a compact family car, 35–45 MPG is good; below 30 MPG suggests an older or larger engine. For SUVs and pickup trucks, 20–28 MPG is typical. For hybrids, 45–60 MPG is common. "Good" is relative to vehicle category and driving pattern — a large 4x4 getting 25 MPG in genuine off-road use may be performing efficiently for its size.