diff --git a/src/lib/subs/attsub.c b/src/lib/subs/attsub.c
index 4248f19e..955cdd24 100644
--- a/src/lib/subs/attsub.c
+++ b/src/lib/subs/attsub.c
@@ -856,7 +856,7 @@ calc_mobcost(int combat_mode, struct combat *off, struct combat *def,
 	case EF_SHIP:
 	    /* the 2 in the formula below is a fudge factor */
 	    getship(def->shp_uid, &ship);
-	    off->mobcost += (def->eff / 100) * (shp_speed(&ship) / 2);
+	    off->mobcost += shp_speed(&ship) / 2 * def->eff / 100;
 	}
     }
 }
diff --git a/tests/retreat/final.xdump b/tests/retreat/final.xdump
index 874d35b0..aa32bc4f 100644
--- a/tests/retreat/final.xdump
+++ b/tests/retreat/final.xdump
@@ -30,7 +30,7 @@ uid owner xloc yloc type effic mobil off tech opx opy mission radius fleet civil
 2 1 -3 1 10 93 118 0 45 0 0 none 0 "" 0 9 0 0 0 0 0 0 0 0 0 0 0 0 healthy 0 0 "" -3 1 1 (injured torpedoed sonared helpless bombed depth-charged boarded) "bgyujnbgy"
 3 1 -3 1 10 100 127 0 45 0 0 none 0 "a" 0 10 0 0 0 0 0 0 0 0 0 0 0 0 healthy 0 0 "" -3 1 1 () ""
 4 1 -5 1 10 100 127 0 45 0 0 none 0 "a" 0 10 0 0 0 0 0 0 0 0 0 0 0 0 healthy 0 0 "" -3 1 1 (group helpless) "b"
-5 1 -5 1 17 96 63 0 70 0 0 none 0 "" 0 6 31 1 0 0 0 0 0 0 0 0 0 0 healthy 0 0 "" -3 1 1 () ""
+5 1 -5 1 17 96 61 0 70 0 0 none 0 "" 0 6 31 1 0 0 0 0 0 0 0 0 0 0 healthy 0 0 "" -3 1 1 () ""
 6 1 -1 3 18 100 97 0 165 0 0 none 0 "" 0 10 21 1 0 0 0 0 0 0 0 0 0 0 healthy 0 0 "" -3 1 1 () ""
 7 1 -5 1 10 100 127 0 45 0 0 none 0 "a" 0 10 0 0 0 0 0 0 0 0 0 0 0 0 healthy 0 0 "" -3 1 1 (group helpless) "b"
 9 2 -4 -2 17 60 127 0 70 0 0 none 0 "" 0 60 38 1 0 0 0 0 0 0 0 0 0 0 healthy 0 0 "" -3 1 1 () ""
diff --git a/tests/retreat/journal.log b/tests/retreat/journal.log
index fee3e622..16343a97 100644
--- a/tests/retreat/journal.log
+++ b/tests/retreat/journal.log
@@ -1548,7 +1548,7 @@
     Play#0 output Play#0 1   1    2 lc   light cruis   -3,1        93%   0   9   0   0  0  0  0  0 118   45
     Play#0 output Play#0 1   1    3 lc   light cruis   -3,1    a  100%   0  10   0   0  0  0  0  0 127   45
     Play#0 output Play#0 1   1    4 lc   light cruis   -5,1    a  100%   0  10   0   0  0  0  0  0 127   45
-    Play#0 output Play#0 1   1    5 dd   destroyer     -5,1        96%   0   6   0   0  0  0  0  0  63   70
+    Play#0 output Play#0 1   1    5 dd   destroyer     -5,1        96%   0   6   0   0  0  0  0  0  61   70
     Play#0 output Play#0 1   1    6 sb   submarine     -1,3       100%   0  10   0   0  0  0  0  0  97  165
     Play#0 output Play#0 1   1    7 lc   light cruis   -5,1    a  100%   0  10   0   0  0  0  0  0 127   45
     Play#0 output Play#0 1   2    9 dd   destroyer     -4,-2       60%   0  60   0   0  0  0  0  0 127   70