for Google Sheets

FuelEconomyConversion.txt

=if(unit_index=unit_display_index,
  fe_value,
  if(unit_index=1,
    let(
      fe_value_in_kpl, 100/fe_value,
      if(unit_display_index=2,
        fe_value_in_kpl*KPLinMPGus,
        if(unit_display_index=3,
          fe_value_in_kpl*KPLinMPGi,
          fe_value_in_kpl
        )
      )
    ),
    if(unit_index=2,
      let(
        fe_value_in_kpl, fe_value/KPLinMPGus,
        if(unit_display_index=1,
          100/fe_value_in_kpl,
          if(unit_display_index=3,
            fe_value*MPGusinMPGi,
            fe_value_in_kpl
          )
        )
      ),
      if(unit_index=3,
        let(
          fe_value_in_kpl, fe_value/KPLinMPGi,
          if(unit_display_index=1,
            100/fe_value_in_kpl,
            if(unit_display_index=2,
              fe_value/MPGusinMPGi,
              fe_value_in_kpl
            )
          )
        ),
        if(unit_display_index=1,
          100/fe_value,
          fe_value*if(unit_display_index=2,
            KPLinMPGus,
            KPLinMPGi
          )
        )
      )
    )
  )
)