In the fourth (?) verse of Jingle Bells there's a reference to a fast horse, "two-forty as his speed?" What's that mean? 240 hoofbeats per minute? Something else?

Now the ground is white,
Go it while you're young,
Take the girls along
And sing this sleighing song.
Just get a bob-tailed bay,
Two-forty as his speed,
Hitch him to an open sleigh
and crack! You'll take the lead.