system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some trucks have a spare set of wheels hanging? Wouldn't it better to just have all the wheels on the road?
gpt
Having the extra set of wheels down adds transmission drag (and thus reduces fuel economy) and also causes more damage to the tyres when turning, so unless you absolutely need them to support the weight of the load, it's better to have one out of the way.