My truck came with 285 65 20 tires and the spare obviously should not be used as its so much smaller 245-70-17 285 75 17 tire is around the same size as tires can I mount that on that rim or is it a special spare tire rim and will it fit under the truck

 

You can mount the larger tire on the spare rim. I suspect the spare's rim is 7" wide.

 

The spare CAN be used and is just fine to get you by until your tire is repaired/replaced

Per p. 547 of the manual: Dissimilar Spare Wheel and Tire Assembly Information: If you have a dissimilar spare wheel and tire, the intent of the spare wheel is for temporary use only. This means that if you need to use it, replace it as soon as possible with a road wheel and tire assembly that is the same size and type as the road tires and wheels originally provided by Ford.

Using 4 wheel drive (p 270 of the manual) or your E-Locker (p. 279) is a concern

 

I am considering putting 17 inch Raptor SVT wheels on with some Bridgestone 255/75r17 tires load rated at 111. 111 is the stock load rating per my door sticker and the size will ensure I dont have speedometer calibration issues. The tall tires with 111 load rating will soften the ride considerable compared to the 20 inch wheels and tires I have now. The spare will be compatible as is as well. I am retired and dont do any heavy hauling or towing.