Tyre Size help

Hi

anyone a whizz with tyre sizes

i have 235/70 x 16 on it at the moment
looking to fit some slightly different alloys as the ones on the motor are not great and want to save the BF Goodrich for bad weather and muddy roads and tracks around the farm

so have the offer of some 17" rims of a disco 3
these are currently fitted with 235/70 x 17

what size tyres do i need to fit to the 17" rims to get them to the same height as mine so that they will not affect the speedo being correct and the gearing stayinbg the same

also do i need spigot rings with these

http://www.kouki.co.uk/utilities/visual-tyre-size-calculator

Try that mate

235 65 17 (231.63 rolling diameter)
would be as close as you could get to 235 70 16 (231.03 Rolling diameter)

Cheers for that

that website is awesome

many thanks for your help, now had better start looking for tyres

Ian