diff --git a/APMrover2/mode.cpp b/APMrover2/mode.cpp
index 0d7a275d2c..e7e1ac1e18 100644
--- a/APMrover2/mode.cpp
+++ b/APMrover2/mode.cpp
@@ -225,7 +225,8 @@ bool Mode::stop_vehicle()
 }
 
 // estimate maximum vehicle speed (in m/s)
-float Mode::calc_speed_max(float cruise_speed, float cruise_throttle)
+// cruise_speed is in m/s, cruise_throttle should be in the range -1 to +1
+float Mode::calc_speed_max(float cruise_speed, float cruise_throttle) const
 {
     float speed_max;
 
diff --git a/APMrover2/mode.h b/APMrover2/mode.h
index a016f19c15..e42f7bed7f 100644
--- a/APMrover2/mode.h
+++ b/APMrover2/mode.h
@@ -121,7 +121,8 @@ protected:
     bool stop_vehicle();
 
     // estimate maximum vehicle speed (in m/s)
-    float calc_speed_max(float cruise_speed, float cruise_throttle);
+    // cruise_speed is in m/s, cruise_throttle should be in the range -1 to +1
+    float calc_speed_max(float cruise_speed, float cruise_throttle) const;
 
     // calculate pilot input to nudge speed up or down
     //  target_speed should be in meters/sec