Woodzy

Any body know what the true tyre size is from factory when i bought my landy it came on 205/65/16 but they were road tyres so i put some 235/75/16 on and my dad was saying that it will affect the speedo, so the cheeky reply to that was when the tread wears down it will go back to normal , I cant see there being much difference from ratios ,and if need be where is the best place to get a transducer to correct the speedo.