EG...
A 3.00 tyre is 3 inches tall, 3 inches wide.
A 3.50 tyre is 3.5 inch wide, 3.5 inches tall. Top, and bottom. In both cases you add the 8 inch wheel.
Metric tyres are different.
A 100/80 - 8 is:
100mm wide. (4 inches)
80% of 100 tall so 80mm tall top and bottom. (3.5 inches)
A 90/80 tyre is then...
90 wide or 3.75 inches
And 80% of 90 tall... OR about 78mm or 3 and 1/8 and a bit inches.
So this last tyre would be a little wider than a a 3.50 at 3.75. And the same height (tiny fraction bigger) as a stock 3.00 tyre.
EG https://www.mobilitypitstop.com/product ... oter-tyre/
And there are others. But ideally it needs a slightly wider rim. At around 2.75 inches. It will FIT your stock rim but will look narrower and be pinched in slightly.
Or 80/80 http://www.ebay.co.uk/itm/80-80-x-8-Bla ... 1674257970 is 3.15 wide and slightly smaller diameter than a 3.00 tyre...