diff --git a/changes/ticket40337 b/changes/ticket40337
new file mode 100644
index 0000000000..1c86fc4c99
--- /dev/null
+++ b/changes/ticket40337
@@ -0,0 +1,16 @@
+  o Minor features (testing):
+    - On a testing network, relays can now use the
+      TestingMinTimeToReportBandwidth option to change
+      the smallest amount of time over which they're willing to report
+      their observed maximum bandwidth.  Previously, this was fixed
+      at 1 day.  For safety, values under 2 hours are only supported on
+      testing networks. Part of a fix for ticket 40337.
+
+  o Minor features (testing):
+    - Relays on testing networks now report their observed bandwidths
+      immediately from startup.  Previously, they waited
+      until they had been running for a full day.   Closes ticket
+      40337.
+    - Relays on testing networks no longer rate-limit how frequently
+      they are willing to report new bandwidth measurements. Part of a fix
+      for ticket 40337.
diff --git a/doc/man/tor.1.txt b/doc/man/tor.1.txt
index 7222cd0548..89afe59582 100644
--- a/doc/man/tor.1.txt
+++ b/doc/man/tor.1.txt
@@ -3597,6 +3597,11 @@ The following options are used for running a testing Tor network.
     Minimum value for the Fast flag.  Overrides the ordinary minimum taken
     from the consensus when TestingTorNetwork is set. (Default: 0.)
 
+[[TestingMinTimeToReportBandwidth]] **TestingMinTimeToReportBandwidth** __N__ **seconds**|**minutes**|**hours**::
+    Do not report our measurements for our maximum observed bandwidth for any
+    time period that has lasted for less than this amount of time.
+    Values over 1 day have no effect. (Default: 1 day)
+
 [[TestingServerConsensusDownloadInitialDelay]] **TestingServerConsensusDownloadInitialDelay** __N__::
     Initial delay in seconds for when servers should download consensuses. Changing this
     requires that **TestingTorNetwork** is set. (Default: 0)
diff --git a/src/app/config/config.c b/src/app/config/config.c
index bfa258c904..2877bc1e6a 100644
--- a/src/app/config/config.c
+++ b/src/app/config/config.c
@@ -550,6 +550,7 @@ static const config_var_t option_vars_[] = {
   V(MaxUnparseableDescSizeToLog, MEMUNIT, "10 MB"),
   VPORT(MetricsPort),
   V(MetricsPortPolicy,           LINELIST, NULL),
+  V(TestingMinTimeToReportBandwidth,    INTERVAL, "1 day"),
   VAR("MyFamily",                LINELIST, MyFamily_lines,       NULL),
   V(NewCircuitPeriod,            INTERVAL, "30 seconds"),
   OBSOLETE("NamingAuthoritativeDirectory"),
@@ -3980,6 +3981,7 @@ options_validate_cb(const void *old_options_, void *options_, char **msg)
     CHECK_DEFAULT(TestingSigningKeySlop);
     CHECK_DEFAULT(TestingAuthKeySlop);
     CHECK_DEFAULT(TestingLinkKeySlop);
+    CHECK_DEFAULT(TestingMinTimeToReportBandwidth);
     or_options_free(dflt_options);
   }
 #undef CHECK_DEFAULT
diff --git a/src/app/config/or_options_st.h b/src/app/config/or_options_st.h
index 510ece42a3..151b77c457 100644
--- a/src/app/config/or_options_st.h
+++ b/src/app/config/or_options_st.h
@@ -1060,6 +1060,10 @@ struct or_options_t {
   /** List of policy allowed to query the Metrics port. */
   struct config_line_t *MetricsPortPolicy;
 
+  /** How far must we be into the current bandwidth-measurement period to
+   * report bandwidth observations from this period? */
+  int TestingMinTimeToReportBandwidth;
+
   /**
    * Configuration objects for individual modules.
    *
diff --git a/src/app/config/testnet.inc b/src/app/config/testnet.inc
index 00b307782b..039454a0d0 100644
--- a/src/app/config/testnet.inc
+++ b/src/app/config/testnet.inc
@@ -19,6 +19,7 @@
 { "TestingV3AuthInitialDistDelay", "20 seconds" },
 { "TestingAuthDirTimeToLearnReachability", "0 minutes" },
 { "MinUptimeHidServDirectoryV2", "0 minutes" },
+{ "TestingMinTimeToReportBandwidth", "0 seconds" },
 { "TestingServerDownloadInitialDelay", "0" },
 { "TestingClientDownloadInitialDelay", "0" },
 { "TestingServerConsensusDownloadInitialDelay", "0" },
diff --git a/src/feature/relay/router.c b/src/feature/relay/router.c
index 67d3e3ee75..a2ca472307 100644
--- a/src/feature/relay/router.c
+++ b/src/feature/relay/router.c
@@ -2622,7 +2622,10 @@ check_descriptor_bandwidth_changed(time_t now)
   if ((prev != cur && (!prev || !cur)) ||
       cur > (prev * BANDWIDTH_CHANGE_FACTOR) ||
       cur < (prev / BANDWIDTH_CHANGE_FACTOR) ) {
-    if (last_changed+MAX_BANDWIDTH_CHANGE_FREQ < now || !prev) {
+    const bool change_recent_enough =
+      last_changed+MAX_BANDWIDTH_CHANGE_FREQ < now;
+    const bool testing_network = get_options()->TestingTorNetwork;
+    if (change_recent_enough || testing_network || !prev) {
       log_info(LD_GENERAL,
                "Measured bandwidth has changed; rebuilding descriptor.");
       mark_my_descriptor_dirty("bandwidth has changed");
diff --git a/src/feature/stats/bwhist.c b/src/feature/stats/bwhist.c
index d5a9b73605..552dc7ad74 100644
--- a/src/feature/stats/bwhist.c
+++ b/src/feature/stats/bwhist.c
@@ -206,16 +206,24 @@ bwhist_note_dir_bytes_read(uint64_t num_bytes, time_t when)
   add_obs(dir_read_array, when, num_bytes);
 }
 
-/** Helper: Return the largest value in b->maxima.  (This is equal to the
+/**
+ * Helper: Return the largest value in b->maxima.  (This is equal to the
  * most bandwidth used in any NUM_SECS_ROLLING_MEASURE period for the last
  * NUM_SECS_BW_SUM_IS_VALID seconds.)
+ *
+ * Also include the current period if we have been observing it for
+ * at least min_observation_time seconds.
  */
 STATIC uint64_t
-find_largest_max(bw_array_t *b)
+find_largest_max(bw_array_t *b, int min_observation_time)
 {
   int i;
   uint64_t max;
-  max=0;
+  time_t period_start = b->next_period - NUM_SECS_BW_SUM_INTERVAL;
+  if (b->cur_obs_time > period_start + min_observation_time)
+    max = b->max_total;
+  else
+    max = 0;
   for (i=0; i<NUM_TOTALS; ++i) {
     if (b->maxima[i]>max)
       max = b->maxima[i];
@@ -233,8 +241,9 @@ MOCK_IMPL(int,
 bwhist_bandwidth_assess,(void))
 {
   uint64_t w,r;
-  r = find_largest_max(read_array);
-  w = find_largest_max(write_array);
+  int min_obs_time = get_options()->TestingMinTimeToReportBandwidth;
+  r = find_largest_max(read_array, min_obs_time);
+  w = find_largest_max(write_array, min_obs_time);
   if (r>w)
     return (int)(((double)w)/NUM_SECS_ROLLING_MEASURE);
   else
diff --git a/src/feature/stats/bwhist.h b/src/feature/stats/bwhist.h
index e7fc60fdee..d61c442e5d 100644
--- a/src/feature/stats/bwhist.h
+++ b/src/feature/stats/bwhist.h
@@ -28,7 +28,7 @@ int bwhist_load_state(struct or_state_t *state, char **err);
 
 #ifdef BWHIST_PRIVATE
 typedef struct bw_array_t bw_array_t;
-STATIC uint64_t find_largest_max(bw_array_t *b);
+STATIC uint64_t find_largest_max(bw_array_t *b, int min_observation_time);
 STATIC void commit_max(bw_array_t *b);
 STATIC void advance_obs(bw_array_t *b);
 STATIC bw_array_t *bw_array_new(void);
diff --git a/src/test/test_relay.c b/src/test/test_relay.c
index 7338340e25..dbedc021e4 100644
--- a/src/test/test_relay.c
+++ b/src/test/test_relay.c
@@ -98,7 +98,7 @@ test_relay_close_circuit(void *arg)
   tt_int_op(new_count, OP_EQ, old_count + 1);
 
   /* Ensure our write totals are 0 */
-  tt_u64_op(find_largest_max(write_array), OP_EQ, 0);
+  tt_u64_op(find_largest_max(write_array, 86400), OP_EQ, 0);
 
   /* Mark the circuit for close */
   circuit_mark_for_close(TO_CIRCUIT(orcirc), 0);
@@ -107,7 +107,7 @@ test_relay_close_circuit(void *arg)
   advance_obs(write_array);
   commit_max(write_array);
   /* Check for two cells plus overhead */
-  tt_u64_op(find_largest_max(write_array), OP_EQ,
+  tt_u64_op(find_largest_max(write_array, 86400), OP_EQ,
                              2*(get_cell_network_size(nchan->wide_circ_ids)
                                 +TLS_PER_CELL_OVERHEAD));