Tesla's electric semi truck looks cheaper to run than diesel over its life, which is good for Tesla and for trucking fleets but bad for diesel truck makers.

• The electric semi pays back its higher upfront cost in about five years, while the trucks last 10 to 15 years.
• Lower service and maintenance costs add to the savings on top of fuel.
• Even the most efficient modern diesel trucks lose on cost per mile, helped by the Tesla's aerodynamics and build.
• The whole case rests on diesel staying expensive — if diesel prices crash, fleets have less reason to pay more upfront.

Outlook: Fleets are expected to switch quickly as long as diesel stays costly, making the truck market a real growth lane for Tesla.