diff --git a/scripts/speed_reader.py b/scripts/speed_reader.py
index a075d5d..d3b3806 100644
--- a/scripts/speed_reader.py
+++ b/scripts/speed_reader.py
@@ -7,7 +7,7 @@ reader = Button(4, pull_up=False, bounce_time=0.0005)
 set_variable("speed_reader_last_impulse_time", datetime.datetime.now(), False)
 set_variable("actual_rpm_unfiltered", 0, False)
 set_variable("impulses", 0, False)
-set_variable("last_impulse_time", datetime.datetime.now(), False)
+set_variable("last_calc_time", datetime.datetime.now(), False)
 
 
 def event():
@@ -31,7 +31,9 @@ def new_event():
 
 def speed_calc():
     now = datetime.datetime.now()
-    last_time = datetime.datetime(get_variable("last_impulse_time"))
+    last_time = get_variable("last_calc_time")
+    set_variable("last_calc_time", datetime.datetime.now(), False)
+    
     impulses = int(get_variable("impulses"))
     set_variable("impulses", 0, False)
     time_delta = (now - last_time).total_seconds()