Horseshoes of speed exist. Build a spell based on them.Also, I'm pretty sure Haste and other similar spells can target steeds, so a cart could move faster if you target the horse pulling it. Doesn't help with things like Move Without Trace, though since the cart still makes tracks. That specific issue has come up in, at least, two games I've played.
I think it's reasonable to create your own spells. I kind of wish the game had some good guidelines for doing so in order to keep them balanced. But I'd think it would be fine if you could take an existing spell and modify it slightly. Fly, for example, let it target an object instead of a person, increase the level, and give it a fly speed equal to a slow moving cart. Higher levels of the spell can be faster so, in the end, you can have a flying chariot.
Making 'Mass Travel' spells that target whole ships have interesting implications to a campaign's economy.
A 'folding' cart that lets you bring your cart into a cave or other small space with you.
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