--- a/dom/media/webspeech/synth/speechd/SpeechDispatcherService.cpp
+++ b/dom/media/webspeech/synth/speechd/SpeechDispatcherService.cpp
@@ -12,16 +12,19 @@
 #include "nsEscape.h"
 #include "nsISupports.h"
 #include "nsPrintfCString.h"
 #include "nsReadableUtils.h"
 #include "nsServiceManagerUtils.h"
 #include "nsThreadUtils.h"
 #include "prlink.h"
 
+#include <math.h>
+#include <stdlib.h>
+
 #define URI_PREFIX "urn:moz-tts:sapi:"
 
 // Some structures for libspeechd
 typedef enum {
   SPD_EVENT_BEGIN,
   SPD_EVENT_END,
   SPD_EVENT_INDEX_MARK,
   SPD_EVENT_CANCEL,
@@ -469,26 +472,21 @@ SpeechDispatcherService::Speak(const nsA
 
   spd_set_synthesis_voice(mSpeechdClient,
                           NS_ConvertUTF16toUTF8(voice->mName).get());
 
   // We provide a volume of 0.0 to 1.0, speech-dispatcher expects 0 - 100.
   spd_set_volume(mSpeechdClient, static_cast<int>(aVolume * 100));
 
   // We provide a rate of 0.1 to 10 with 1 being default.
-  // speech-dispatcher expects -100 to 100 with 0 being default.
-  int rate = 0;
+  // The rate range in speechd is roughly x3 at both ends (0.334 .. 3).
+  int rate = std::max<float>(std::min<float>(aRate, 3), 0.334);
 
-  if (aRate > 1) {
-    rate = static_cast<int>((aRate - 1) * 10);
-  } else if (aRate <= 1) {
-    rate = static_cast<int>((aRate - 1) * (100/0.9));
-  }
-
-  spd_set_voice_rate(mSpeechdClient, rate);
+  // speech-dispatcher expects -100 to 100 with 0 being default.
+  spd_set_voice_rate(mSpeechdClient, log10(rate) / log10(3) * 100);
 
   // We provide a pitch of 0 to 2 with 1 being the default.
   // speech-dispatcher expects -100 to 100 with 0 being default.
   spd_set_voice_pitch(mSpeechdClient, static_cast<int>((aPitch - 1) * 100));
 
   // The last three parameters don't matter for an indirect service
   nsresult rv = aTask->Setup(callback, 0, 0, 0);