How to measure your real MPG tank to tank
A reliable fill-to-fill method for measuring true fuel economy, why the trip computer often disagrees, and how to compare US MPG with L/100km.
The fill-to-fill method that actually works
The most dependable way to measure economy is to fill the tank completely, reset the trip odometer, drive normally, then fill completely again at the same pump style. The pump tells you the fuel added, which is exactly the fuel you burned since the last fill, and the trip odometer tells you the distance. Feed those two numbers into this calculator and you get a real MPG rather than an estimate. One tank can be noisy, so repeating over three or four fills and averaging smooths out how far the auto-shutoff clicked each time.
Why the trip computer disagrees
Dashboard fuel economy readouts are convenient but they are modelled estimates, not direct measurements, and they commonly read a few percent optimistic. They infer fuel flow from injector timing and other sensors rather than measuring litres pumped, so calibration drift and rounding creep in. A tank-to-tank calculation using pump gallons and odometer miles is grounded in the two quantities that actually matter. If your measured figure sits below the trip computer by a small margin, that gap is normal and the measured number is the one to trust.
Reading MPG next to L/100km
This tool shows US MPG, litres per 100 km and kilometres per litre together, which helps when comparing cars across regions. MPG and km/L rise as a car gets thriftier, so bigger is better, while L/100km falls, so smaller is better. Because MPG is an inverse of fuel per distance, equal steps in MPG do not mean equal steps in fuel saved, which is why the L/100km column is often the clearer one for judging real differences. Seeing all three at once removes the guesswork of mental conversion.
Turning economy into cost per mile
Once you know the fuel used, adding the pump price converts economy into money. The total fuel cost is simply fuel times price, and cost per mile divides that by the distance, giving a figure you can multiply by any future trip length. This is handy for budgeting a road trip or comparing two routes of different length. Because the number is driven by both economy and price, a cheaper fill or a lighter foot on the throttle both push the cost per mile down.