120/70 8 is same diameter.
Or 130/70 8?
110/80 or 100/80 also options.
But all need a wider rim.
You work it out like this:
3.50 x 8 is: 3.5+3.5 (top bottom sidewalls) + 8 rim. So 15 diameter.
120/80 for eg means 120 wide, but 80% of this sidewall. So each sidewall is 80% of 120,,. Which is 96mm top, + bottom.
Thats 192 div by 25.4 to give inches. + the 8 inches rim diameter. So about 15.8 inches. So a bit bigger.
Same diameter as my 130/60 10 or 145/50 10 tyres. A ride height increase of only 12mm unless you lower the pressure some.
These are the same diameter, but need a 3.00 or 3.25 to 3.5 wide rim.
http://www.shirebikes.co.uk/collections ... eless-tyreFitted
http://i74.servimg.com/u/f74/11/66/30/96/img_2926.jpgSame. Fitted to a WIDE rim,
http://www.gm-moto.jp/item/duro8120/ 4.75 inches.