Just bought an 06 Kings Rance SCrew with the optional 20 inch wheels and Pirelli Scorpion Tires. But when I checked the spare, it is a Michelin 18" wheel and tire, can this be correct??? a $970 option, and Ford gives you an 18" spare, with a different brand of tire. So much for 5 tire rotation, huh?
Plus I have 3:70 limited slip differential, and I know both rear tires have to be the same, or it throws the Posi all to hell.

 

As long as the overal diameter is the same, its fine.
I wouldnt want to have a 5th spare be a usable wheel and tire, not when its stored under the truck like it is. Not to mention, Ford has always used the plain steel wheels for spares, no matter what wheels you have.

 

I looked in the Ford website and all the 20" tires indicate (4) which means 4 tires of that size. The spare is not the same size, so that's the way your truck came from the factory.

[Ford Link]
http://www.fordvehicles.com/trucks/f150/features/

 

The tire size matters, not the rim size. 275/55R20 = 31.9" diameter. 275/65R18 = 32.1" tire. 0.52% difference. i wouldn't worry.

FYI, no one rotates 5 tires any more. Front to back, back to front is all you should need to do with most vehicles. Unless you only turn left or right, most people wear both sides evenly.

 

Sorry, I am a little slow. does that mean that 275 X 65 X 18 are actually taller than 275 X 55 X 20? Thanks

 

Yep, by, 2 tenths of an inch.

 

Yes by .2", which is not enough to cause any problems.

 

The factory limited slip (trac-loc) is junk anyway. Plus, limited slip doens't fair all that well with trucks because of the larger tire size... it's way too involved to explain, but DTS (drive train specialists) gave me the complete rundown on it when I had a new trac-loc installed in my front diff, with an eaton E-Locker in my rear and 4.10 gears. 2 tenths of an inch is really minimal, and for a spare you'll be just fine. That doesn't mean however that you should drive your truck 10,000miles on the spare... think of it like this... for about every 1000 miles you would drive wth your spare on, it would travel an additional 5 miles given it's about .5% taller; that's added friction that your limited slip doesn't need. But we're talking extreams here... just be senseable and get your tire replaced as soon as possible. I would think if you can afford the truck you bought that you could afford to have a tire repaired, or replaced (usually best to replace in pairs) as opposed to driving for months on end using a spare.