How to measure real MPG
- Fill the tank until the pump clicks off the first time. Reset the trip meter.
- Drive normally until you need fuel again.
- Fill up the same way (first click, ideally the same pump) and note the gallons.
- MPG = miles on the trip meter ÷ gallons you just put in.
One tank is noisy. Average three or more tanks for a number you can trust, especially after a mod.
Why you might not match the EPA number
Short trips hurt most. A cold engine runs rich until it warms up, so a car that only does 3-mile errands can land well under its rating.
Then speed. Aero drag goes up with the square of speed, so at 80 mph the car is pushing about 50% more air resistance than at 65.
Wider, stickier or heavier tires add rolling resistance. A taller tire also makes the odometer read short, so your MPG looks worse than it really is. The tire size calculator tells you by how much.
If you've lost a few MPG over a year without changing how you drive, check tire pressure, dragging brakes and old O2 sensors first.
Converting to L/100 km
L/100 km = 235.215 ÷ mpg (US gallons). Unlike MPG, a lower number is better.