What is the best background (colour) to take accurate/consistant tacho readings. I have a wooden fence in my back garden and get very different readings depending on what time of day it is and the differing weather conditions.

Anyone??

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.

Would a light source behind the model make for more consistant readings then??

It should, but I've found that nothing beats sunlight. Most 'artificial' light sources I found, are too weak.

ASlo check your batteries. I would suspect that performance will deminish with decaying batteries

Keep in mind that at different times of the day, and in differnet weather conditions, the engine will actually run at different RPM, sometimes by quite a lot. Temperature and humidity have big effects on engine performance, and it's not uncommon to get noticeable RPM changes during the day.