diff --git a/pyModeS/decoder/bds/bds06.py b/pyModeS/decoder/bds/bds06.py
index 2c366ce..bd7a8f8 100644
--- a/pyModeS/decoder/bds/bds06.py
+++ b/pyModeS/decoder/bds/bds06.py
@@ -168,13 +168,11 @@ def surface_velocity(msg, source=False):
     elif mov == 124:
         spd = 175
     else:
-        movs = [2, 9, 13, 39, 94, 109, 124]
-        kts = [0.125, 1, 2, 15, 70, 100, 175]
-        i = next(m[0] for m in enumerate(movs) if m[1] > mov)
-        step = (kts[i] - kts[i - 1]) * 1.0 / (movs[i] - movs[i - 1])
-        spd = kts[i - 1] + (mov - movs[i - 1]) * step
-        spd = round(spd, 2)
-
+        mov_lb = [2, 9, 13, 39, 94, 109, 124]
+        kts_lb = [0.125, 1, 2, 15, 70, 100, 175]
+        step = [0.125, 0.25, 0.5, 1, 2, 5]
+        i = next(m[0] for m in enumerate(mov_lb) if m[1] > mov)
+        spd = kts_lb[i - 1] + (mov - mov_lb[i - 1]) * step[i - 1]
     if source:
         return spd, trk, 0, "GS", "TRUE_NORTH", None
     else:
diff --git a/tests/test_adsb.py b/tests/test_adsb.py
index c6e6de0..2b8b631 100644
--- a/tests/test_adsb.py
+++ b/tests/test_adsb.py
@@ -76,7 +76,7 @@ def test_adsb_velocity():
     vgs_surface = adsb.velocity("8FC8200A3AB8F5F893096B000000")
     assert vgs == (159, 182.88, -832, "GS")
     assert vas == (375, 243.98, -2304, "TAS")
-    assert vgs_surface == (19.0, 42.2, 0, "GS")
+    assert vgs_surface == (19, 42.2, 0, "GS")
     assert adsb.altitude_diff("8D485020994409940838175B284F") == 550