I'm aware of Colonial era British Navy "boarding grenades" which were about 3.3" in diameter, and Rev War Colonial Army grenades which were about 3.53" in diameter... but no 2.5" grenades from that era. However, there was a civil war Confederate round-ball grenade which was 2.5" in diameter. One is shown on page 500 (in the Grenades section) of my book "Field Artillery Projectiles Of The American Civil War."
Establishing whether or not your ball is a Confederate grenade will of course require a high standard of proof. You'll have to clean it well enough to get a highly accurate measurement of its original iron surface's diameter, not counting any rust-encrustation buildup. After the encrustation is cleaned off, weigh it on a super-accurate scale, such as a Postal Shipping scale, which measures in 1/10th-ounce increments. If it then matches up with the known diameter and exact weight of the Confederate ball-grenade, you'll have proved its identity. If it doesn't match, it's a tractor's gearshift knob or something similar.
Actually, as a possible time-&-effort saver, you could weigh it on a Postal Shipping scale first. If it is a solid ball which has a mounting-hole drilled in it, such as a gearshift knob, its precise weight will give us that identity. If its weight shows it is a hollow ball, then the identity-quest gets a lot more interesting.