Estimate stories using a distance measure. Furlongs for fun, miles if you're a boring American, kilometers if you're a boring person on the rest of the planet.
Your team velocity is measured in furlongs-per-sprint. This makes clear the separation between how much work needs to be done and how fast a team can complete it. It calls out the knobs that management can turn immediately: go faster, do less work.
For the vehicle, consider a stagecoach. Every new horse added to a stagecoach adds to its potential speed. Four horses is better and faster than one horse, but not four times better. There's a point where you're better off with two stagecoaches.
Breakdowns:
Another popular analogy is rocks-in-a-box. Your velocity is called capacity, and described as a box for the Product Owner to fill with rocks. Rock size is estimated by the team. Lots of small rocks do a better job of filling up the box. This point, that small well-understood stories are easier to complete, is missing from the stagecoach analogy. (If the miles are short, they go by faster? Doesn't quite work.)
