Looking under my new Lariat I see that the spare tire is a different size than the tires on the truck. When we placed the order as soon as the 5th wheel hitch was selected the tire size changed to LT275/65 R20, 3750lbs at 80psi. The spare tire is LT275/70 R18, 3640lbs at 80psi. Going to call the dealer today & see what the story is. Anyone else notice this?
John

 

Pretty standard practice. You need to be concerned about the over all diameter of the combination, not the stamped size of the tire.

In your case the width is the same between the two tires and the over all diameter difference is .-91 at least using the tire calculator I used on line

 

Yup, completely normal. Nobody gets a 20" spare tire. The outside diameter should be very close between the two and cause no issues.

 

That's pretty crappy. That's essentially a $200 tire under there that you can't use when you need new tires. I always buy 3 to match the spare when I replace the originals.

 

I do that on my first set and then replace the spare with whatever worn out tire I took off. That way I've got something to get me home if I have a blow out, but not worried about wasting a tire. Having the initial tire off by a bit wouldn't really work that way.

 

Called the dealer about it this morning, he said he would get back to me & just did. The two tires have the same mounted outside diameter & "that's the way Ford builds em". The 20" tire has a weight rating 110lbs higher than the 18" tire. I wasn't thrilled with the answer, but that's the answer.
John

 

Actually, read your manual. The diameter is pretty close but the circumference is still about 1.3 to 1.6 inches different. They can't get or don't want to spend the money on a 20 inch steel rim, and a 20 inch tire is a good bit more as well.

But here's the rub. There's enough circumference difference that the manual limits you to using it on, I believe, the non-drive wheels in two wheel drive only. If you flat a drive wheel, you have to move a non-drive wheel to the drive wheel that flatted and put the spare on the non-drive position. And even then, it's limited to a hundred miles or so.

I went through this on a 2012 and found that I could buy a second hand original Ford alloy rim and put on a 20 inch tire, and it would fit in the spare wheel space. The rim does get beaten up a bit with debris thrown up from driving, but at least it's a fully functional spare. Some people have said that their trucks did not have enough room to accommodate the slightly larger tire. I find that a bit odd, unless it's a feature of short beds (mine is 8'). But it's worth checking. If you are driving rural a lot and may be several hundred miles from a replacement tire, or if you're in 4WD a lot, you may want to look at this seriously. You'd be better off with a full spare bolted into the bed, even if it doesn't fit under the bed.

 

It could be that they are using an aftermarket hitch that fits tighter around the spare.

 

That could be a possibility for some of them at least. The few that I saw all had the factory standard installed Class IV hitch. We only have 8' trucks, but measuring some 6-1/2' beds it seemed that the fuel tank and other things were a little closer to the spare tire space. Even with an 8' bed there isn't a lot of room; it just gets precarious with 6-1/2' or with whatever keeps the tire from fitting. It only misses by a fraction of an inch. I also saw one at a dealership that had the width for the tire, but if we tried to crank up my full spare, it only went up about two thirds of the way and was left hanging low and rocking badly. It wasn't my truck to play with so I couldn't explore more, but it definitely had a problem. The moral is to check out your own truck on a lift when you have a tire off before you go for a full spare, or be prepared to mount and lock it in your truck bed if it doesn't fit.

 

Just checked my spare . 2012 F350 lariat , 4x4, 6.7 with 20" wheels. The spare 18".

 

Yup. So do check the manual. You'd think Ford would make a bigger thing out of it, since the downside can be pretty significant. I'm just glad I've never had to use the spare, even when I've had to limp in on 45 psi to a local Discount Tire to get a puncture patched. I believe there was an option to delete the spare, but it was only about $75 and I think it removed the under-bed hangar as well, thus making it a stupid move and a stupid option even for Ford to list. If anyone needs that 18" spare, I have a brand new one, on the rim. So do a lot of unsuspecting Superduty owners as well, I suspect.

 

I still don't think this is a big deal. I didn't have the 20" rims on my previous Super Duties, but if I had a slightly different size spare would have been the last of my concerns.

 

My 15 lariet had the exact tire under it. 20 Michelin

 

I guess it all depends on how the truck is used. Next spring planning to tow the 5th wheel through the Yukon into Alaska, some pretty rural territory. If I'm limited to using the spare for only 100 miles on a drive axle, for me it's a concern. Guess I'll first have to make sure it fits, them buy a 20" rim & tire for a spare.
John

 

Ford says it is, and if someone has a drive train or suspension issue this will be just another "water in the fuel" kind of warranty exclusion.

Plus, these days, tire dimensions are becoming more and more critical. On AWDs, it's pretty crazy -- tires have to be within 1-2/32" in diameter so if replacing a damaged tire or if encountering uneven wear, one has to shave the offender down to match the diameter of the remaining ones. Even on truck and commercial suspensions, manufacturers are getting very precise about what they require, with the implication that any deviation becomes a warranty exclusion. It isn't just Ford either.