I am playing around with some oversize wheels on the van and came across this useful calculator.. Very helpful if you want to keep the speedometer reading accurate after the switch. The % error is quite incredible with just small tyre size changes... I reckon I could get 90mph at at indicated 70mph provided there is enough torque in 977cc of muscle.
Alternatively 0-60 in about 7secs with a top speed of 45mph !!!
Anyhow heree is the link.. Have fun.
http://www.kouki.co.uk/utilities/visual-tyre-size-calculator Mike