wheel size

can anyone tell me if I change from 15inch to 16inch will this alter my speedo reading, thanks.

If you swap15s with 205/50/15 for 16s with 215/40/16 on the rear (standard wheels and tyres sizes) there is only a very small difference to the Speedo readings.
It will read very slightly higher but you probably wouldn't notice the difference.

Obviously if you put different sized tyres on, the difference will vary but still only marginally. E.g. 215/45 16 tyres will make the Speedo reading very slightly less than the 205/50/15. You are allowed 10% innacuracy.
Hth