Tires Size

I currently have 265 75 17’s on my 98 Expedition and I have an opportunity to buy 315 75 17’s at a good price will they fit?

They’re way bigger, I wouldn’t try it, assuming 265s are the correct size.

As texases said, these tires are MUCH wider than your present tires.
I seriously doubt if they would fit, but you could consult the ultimate authority, which should be sitting in your glove compartment.

Most Owner’s Manuals will list acceptable “alternate” tire sizes for a vehicle. Or, if Ford did not see fit to include this information in the manual, try the Tire Rack website which will list all tire sizes that will fit your Expedition.

The outside diameter and radius are bigger as well…a possible fitment, handling, abs if you have it problems. Too much of a difference IMO.
Look up “powerdog” tire fit calculator to get % difference.

They will likely rub on the fenderwells. Suspension travel will suffer. There will be significant changes to the effective axle ratio. Your vehicle will seems sluggish, your odometer and speedometer will be off. And the transmission will be somewhat erratic since it bases all it’s operations off of the VSS, which in turn is based off of what the speedometer is reading ( it may be the other way around). Anyway unless you plan on lifting your vehicle and regearing it (expect to spend well over $5k), the idea is foolhardy.

They’re 50mm wider. I seriously doubt if the beads will seat right. That’ll affect your handling as a minimum and maybe even your safety. And that’s even if they fit in the wells without compromising articulation. No price break is THAT good.