Tyre sizes for rim widths

[J_T]

Been trying to work out which tyres I can put on my sets of rims.  I have standard 16" 8J RX7 rims and standard 18" 8J RX8 rims.

I\'ve looked at the tyre calculator here: http://www.tyresite.com/tyrecalc.asp (seems to have clones on other sites too)

The rim width calculator section on that will not go up to 8" rims when you input standard FD wheel dimensions (225/50R16 right?) so I\'m not inclined to trust it and prefer to ask for real world experience.

Is there a general rule of thumb for this?

TIA, Jon

[SiH]

Realistically, on an 8 conservative fitments would be anything between 205 and 245
Anything less than that is a slight stretch, more may make it a bit ballony and wobbly.