diff --git a/libraries/AP_ADSB/AP_ADSB.cpp b/libraries/AP_ADSB/AP_ADSB.cpp
index dc0cb16b2e..3ca9af98a9 100644
--- a/libraries/AP_ADSB/AP_ADSB.cpp
+++ b/libraries/AP_ADSB/AP_ADSB.cpp
@@ -958,6 +958,26 @@ bool AP_ADSB::Loc::vertical_accuracy(float &vacc) const
     return true;
 }
 
+uint8_t AP_ADSB::convert_maxknots_to_enum(const float maxAircraftSpeed_knots)
+{
+    if (maxAircraftSpeed_knots <= 0) {
+        // not set or unknown. no bits set
+        return 0;
+    } else if (maxAircraftSpeed_knots <= 75) {
+        return 1;
+    } else if (maxAircraftSpeed_knots <= 150) {
+        return 2;
+    } else if (maxAircraftSpeed_knots <= 300) {
+        return 3;
+    } else if (maxAircraftSpeed_knots <= 600) {
+        return 4;
+    } else if (maxAircraftSpeed_knots <= 1200) {
+        return 5;
+    } else {
+        return 6;
+    }
+}
+
 AP_ADSB *AP::ADSB()
 {
     return AP_ADSB::get_singleton();
diff --git a/libraries/AP_ADSB/AP_ADSB.h b/libraries/AP_ADSB/AP_ADSB.h
index 0ebf1b1ce4..de87f7d01c 100644
--- a/libraries/AP_ADSB/AP_ADSB.h
+++ b/libraries/AP_ADSB/AP_ADSB.h
@@ -207,6 +207,8 @@ public:
     // confirm a value is a valid callsign
     static bool is_valid_callsign(uint16_t octal) WARN_IF_UNUSED;
 
+    static uint8_t convert_maxknots_to_enum(const float maxAircraftSpeed_knots);
+
     // Convert base 8 or 16 to decimal. Used to convert an octal/hexadecimal value
     // stored on a GCS as a string field in different format, but then transmitted
     // over mavlink as a float which is always a decimal.
diff --git a/libraries/AP_ADSB/AP_ADSB_Sagetech_MXS.cpp b/libraries/AP_ADSB/AP_ADSB_Sagetech_MXS.cpp
index 67d3e73e25..39fd88810b 100644
--- a/libraries/AP_ADSB/AP_ADSB_Sagetech_MXS.cpp
+++ b/libraries/AP_ADSB/AP_ADSB_Sagetech_MXS.cpp
@@ -120,7 +120,7 @@ void AP_ADSB_Sagetech_MXS::update()
             (mxs_state.inst.icao != (uint32_t)_frontend.out_state.cfg.ICAO_id_param.get() ||
             mxs_state.inst.emitter != convert_emitter_type_to_sg(_frontend.out_state.cfg.emitterType.get()) ||
             mxs_state.inst.size != _frontend.out_state.cfg.lengthWidth.get() ||
-            mxs_state.inst.maxSpeed != convert_airspeed_knots_to_sg(_frontend.out_state.cfg.maxAircraftSpeed_knots)
+            mxs_state.inst.maxSpeed != (sg_airspeed_t)AP_ADSB::convert_maxknots_to_enum(_frontend.out_state.cfg.maxAircraftSpeed_knots)
             )) {
         last.packet_initialize_ms = now_ms;
         send_install_msg();
@@ -358,7 +358,7 @@ void AP_ADSB_Sagetech_MXS::auto_config_installation()
     mxs_state.inst.sda = sg_sda_t::sdaUnknown;
     mxs_state.inst.emitter = convert_emitter_type_to_sg(_frontend.out_state.cfg.emitterType.get());
     mxs_state.inst.size = (sg_size_t)_frontend.out_state.cfg.lengthWidth.get();
-    mxs_state.inst.maxSpeed = convert_airspeed_knots_to_sg(_frontend.out_state.cfg.maxAircraftSpeed_knots);
+    mxs_state.inst.maxSpeed = (sg_airspeed_t)AP_ADSB::convert_maxknots_to_enum(_frontend.out_state.cfg.maxAircraftSpeed_knots);
     mxs_state.inst.altOffset = 0;         // Alt encoder offset is legacy field that should always be 0.
     mxs_state.inst.antenna = sg_antenna_t::antBottom;
 
@@ -511,7 +511,7 @@ void AP_ADSB_Sagetech_MXS::send_install_msg()
     mxs_state.inst.icao = (uint32_t)_frontend.out_state.cfg.ICAO_id_param.get();
     mxs_state.inst.emitter = convert_emitter_type_to_sg(_frontend.out_state.cfg.emitterType.get());
     mxs_state.inst.size = (sg_size_t)_frontend.out_state.cfg.lengthWidth.get();
-    mxs_state.inst.maxSpeed = convert_airspeed_knots_to_sg(_frontend.out_state.cfg.maxAircraftSpeed_knots);
+    mxs_state.inst.maxSpeed = (sg_airspeed_t)AP_ADSB::convert_maxknots_to_enum(_frontend.out_state.cfg.maxAircraftSpeed_knots);
     mxs_state.inst.antenna = sg_antenna_t::antBottom;
 
     last.msg.type = SG_MSG_TYPE_HOST_INSTALL;
diff --git a/libraries/AP_ADSB/AP_ADSB_uAvionix_MAVLink.cpp b/libraries/AP_ADSB/AP_ADSB_uAvionix_MAVLink.cpp
index bb510a738a..1a574ed23b 100644
--- a/libraries/AP_ADSB/AP_ADSB_uAvionix_MAVLink.cpp
+++ b/libraries/AP_ADSB/AP_ADSB_uAvionix_MAVLink.cpp
@@ -177,22 +177,7 @@ uint8_t AP_ADSB_uAvionix_MAVLink::get_encoded_callsign_null_char()
 
     uint8_t encoded_null = 0;
 
-    if (_frontend.out_state.cfg.maxAircraftSpeed_knots <= 0) {
-        // not set or unknown. no bits set
-    } else if (_frontend.out_state.cfg.maxAircraftSpeed_knots <= 75) {
-        encoded_null |= 0x01;
-    } else if (_frontend.out_state.cfg.maxAircraftSpeed_knots <= 150) {
-        encoded_null |= 0x02;
-    } else if (_frontend.out_state.cfg.maxAircraftSpeed_knots <= 300) {
-        encoded_null |= 0x03;
-    } else if (_frontend.out_state.cfg.maxAircraftSpeed_knots <= 600) {
-        encoded_null |= 0x04;
-    } else if (_frontend.out_state.cfg.maxAircraftSpeed_knots <= 1200) {
-        encoded_null |= 0x05;
-    } else {
-        encoded_null |= 0x06;
-    }
-
+    encoded_null = AP_ADSB::convert_maxknots_to_enum(_frontend.out_state.cfg.maxAircraftSpeed_knots);
 
     if (_frontend.out_state.cfg.rf_capable & ADSB_BITBASK_RF_CAPABILITIES_1090ES_IN) {
         encoded_null |= 0x10;