hmm, I'm not arguein, but the way i see it the diff ratio is 4.3:1 and thats alright with 13" wheels, and lets pull some numbers out of our arse here, and lets say that the 13" rubber is 1.5 metres for one revolution, that goes alright, although somewhat sluggish with the low torque 4k, so we put 17"s on and lets say one revoltion is now 2 metres, so basically to turn the wheels once is a longer distance, so to do this at the same acceleration rate as with the 1.5 metre 13"s you need more torque, or a different diff ratio.
thats the way i see it anyway... :P