Hey Jack its 60% [ 205 x2]+13" against 80%[185x2]+13" works out about 50mm over the diameter which if I can remember was originally around 627mm so your 8% is close enough to correct .When I was younger we would say 1inch of rim size[13-14] was the same as one holden diff ratio making 389-336 much the same as 2"[50mm] in wheel diameter. Cheers Haydn.
I just had to satisfy my own curiosity about the various combinations of rims & tyres discussed, particularly when the choice in 13" is so limited.
I found a few more options that can be used on stock FCs without altering the gearing ratio greatly.
Assuming I've worked this out correctly, the below shows combinations with <10% variation, and some with <1% in blue. It was interesting to see the number of suitable combinations for 15, 16 & 17 inch rims (theoretically).
If the gurus think this could be of value to others I'll post it under General Technical.
