Start with the distance split
If half the miles are city miles and half are highway miles, calculate fuel used for each half. For 100 miles at 20 city MPG and 40 highway MPG, city driving uses 50 ÷ 20 = 2.5 gallons and highway driving uses 50 ÷ 40 = 1.25 gallons.
Divide total miles by total gallons
The trip uses 3.75 gallons, so mixed economy is 100 ÷ 3.75 = 26.67 MPG. The arithmetic average of 20 and 40 would be 30 MPG, which underestimates fuel use in this example. In general, mixed MPG = 1 ÷ (city share ÷ city MPG + highway share ÷ highway MPG).
Why our default uses the published combined rating
MileLedger uses the published EPA combined rating directly. Reconstructing it from rounded display values can introduce rounding differences. The city/highway table is context, not a replacement for the official combined field.
When to use a custom mix
A distance-weighted estimate can help explore a route, but it still uses test-cycle ratings. Actual congestion, speed and weather are not captured just by labeling miles as city or highway. Enter measured MPG in the calculator when you have it.