Rickson Wheels Rear only?

You're going to want a spare that can handle the weight, so plan on buying three. My 275/70R18 tires were the same height as my 245/70R19.5 tires and I originally was thinking along the same lines but decided to buy four 19.5's and use my 18's as spares. If I got a flat on the rear, I would move one of the 19.5's off the front and use an 18" there until I could get a repair or replacement. As others pointed out you need to keep the same diameter front and rear to preserve your 4wd and to allow ABS to work properly. Width does not make a difference.

My F250 with 18" F350 wheels was fine carrying my AF811. Once I added the tongue weight of my 8000 lb enclosed trailer, I was too close to tire and rim capacity and needed the 19.5" upgrade.