IMO, car upgrades give no real benefit when you have an energy system. Instead of counting your clicks as 5x and making you burn out your energy faster, they should give a bonus to the reward you get from clicking (like a 10% bonus per upgrade to money gains).
Actually, it's a 'k' and it stands for 'kilo':
10^3 -> k -> kilo ////
10^6 -> M -> mega ////
10^9 -> G -> giga ////
10^12 -> T -> tera ////
10^15 -> P -> peta ////
10^18 -> E -> exa ////
10^21 -> Z -> zetta ////
10^24 -> Y -> yotta ////
After that, it just repeats prefixes.
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Cars helps a lot when your employees motivation is dropping and you want to work asap