private function getConversionMap()
    {
        $conversion_maps = array(
            $this->speed_conversion_map,
            $this->distance_conversion_map,
            $this->pressure_conversion_map,
        );

        foreach ($conversion_maps as $map) {
            if (array_key_exists($this->unit, $map)) {
                return $map;
            }
        }

        throw new \Exception('Trying to convert unsupported values');
    }