Highway speed misfiring in your 2017 Subaru Crosstrek with a P0301 code pointing to cylinder 1 indicates the boxer engine struggles under load at sustained high RPM. Unlike idle misfires, load-dependent misfires often relate to ignition component breakdown under heat and demand or fuel delivery issues at high consumption rates.
Why Misfires Appear at Highway Speed
Highway driving places continuous demand on the ignition and fuel systems. Ignition coils work harder to fire under cylinder pressure at load. Fuel pumps must maintain volume at high consumption rates. Spark plugs must fire reliably thousands of times per minute. Weak components that work adequately at idle fail under this stress.
Ignition Component Failure
Cylinder 1's ignition coil may have internal breakdown that worsens with heat and demand. Spark plug gap widening from electrode wear requires higher voltage to fire under compression. High-voltage leakage through cracked coil boots becomes significant under load.
Fuel Delivery Issues
A marginally weak fuel pump maintaining pressure at idle may sag at high demand. Dirty injectors restricting flow become inadequate at maximum duty cycle. Fuel filter restriction that doesn't matter at idle becomes problematic at highway consumption.
Cylinder 1 Specifics
In the Crosstrek's boxer configuration, cylinder 1 sits at the front passenger side. Thermal and airflow characteristics at this position may differ from other cylinders, potentially explaining why this cylinder develops issues first.