Tire size change and speedometer reading

[redxlt]

I am sure there is a formula out there to use to calculate the difference in speedometer readings when one changes to a different size tire and wheel. If so what is the formula. Any good/bad things to look for when using larger tires? I currently have 15" deer hoof wheels and tires on my 88 Bronco II . Toying with the idea of larger wheels. And no I am not going off road with it.
Thanks

 

[RotorFX4]

Here's one: http://www.net-comber.com/tirecalc.html and another http://www.4lo.com/4LoCalc.htm

 

[RavoHimself]

theres one in the tech library on this site