diff --git a/src/strava_gear/cli.py b/src/strava_gear/cli.py
index 49f22a4..ec2e31f 100644
--- a/src/strava_gear/cli.py
+++ b/src/strava_gear/cli.py
@@ -4,6 +4,7 @@
 import click
 
 from .core import apply_rules
+from .core import warn_unknown_bikes
 from .input.activities import read_input_csv
 from .input.activities import read_strava_offline
 from .input.rules import read_rules
@@ -54,6 +55,7 @@ def main(rules, csv, strava_database, output, report, tablefmt, show_name, show_
     rules = read_rules(rules, aliases=aliases)
     res = apply_rules(rules, activities)
     reports[report](res, output=output, tablefmt=tablefmt, show_name=show_name, show_first_last=show_first_last)
+    warn_unknown_bikes(rules, activities)
 
 
 if __name__ == "__main__":
diff --git a/src/strava_gear/core.py b/src/strava_gear/core.py
index f7e68fe..f8a8e69 100644
--- a/src/strava_gear/core.py
+++ b/src/strava_gear/core.py
@@ -3,6 +3,7 @@
 from typing import Dict
 from typing import Iterable
 from typing import List
+from warnings import warn
 
 from .data import Result
 from .data import Rule
@@ -59,3 +60,12 @@ def merge_asof(activities: Iterable[Dict], rules: Iterable[Rule]):
             rule_cur, rule_next = rule_next, next(rules, None)
 
         yield activity, rule_cur
+
+
+def warn_unknown_bikes(rules: Rules, activities: List[Dict]) -> None:
+    known_bikes = set(rules.bike_names.keys())
+    used_gear = set(activity['gear_id'] for activity in activities if activity['gear_id'])
+
+    unknown_bikes = rules.all_rule_bike_ids() - known_bikes - used_gear
+    if unknown_bikes:
+        warn(f"Unknown bikes in rules, possibly a typo: {unknown_bikes}")
diff --git a/src/strava_gear/data.py b/src/strava_gear/data.py
index a70a0b4..3c0423b 100644
--- a/src/strava_gear/data.py
+++ b/src/strava_gear/data.py
@@ -157,6 +157,9 @@ class Rules:
     components: List[Component]
     rules: List[Rule]
 
+    def all_rule_bike_ids(self) -> Set[BikeId]:
+        return set(b for r in self.rules for b in r.bikes.keys())
+
 
 @dataclass
 class Usage: