I've seen no difference in regards to backgrounds, but I have seen a difference in location, and time of day. the Tach counts teh 'shadows' of teh spinning prop. The better teh shadows, teh better the reading. So, in a garage at night will not yeilds good results, whereas outside at 12:00 noon is perfect (on a sunny day..) and keep teh tach as close to the spinning prop as safely as possible.