gears from 308s to 355s

[racobra]

i had 308s in my 93 cobra put in 355s just wondered how far off will the speedometer reading be. I had the black 20 tooth gear for speed cable. put in the white one which had 23 gears key word there is had I want to put back in the black one just wondered if i will read slow or fast thanks.I read somewhere the white one the way the cut it it only last like 25,000 mile i only have 28,000 on the car the white speedo gear had maybe 15,000 on it and its shot.