Tires: will their fit????, inflation pressures, tire sizing

Question
My original tires on my Ford explorer sportrax are 255/70 r16. Will either 245/75 r16 or 215/85 r16 fit on the same rim?

Answer
Ken,

The original tires on your vehicle were P metric type tires, which means they are low pressure.  You have not indicated so, but the tire sizing you are suggesting that you go to are LT metric type tires - and they require higher inflation pressures to carry the same load.  

So those tires would be inappropriate for your vehicle, regardless of whether you can mount the tire on the rim or not.