I put 95,000 on my 04 Expy, mostly towing, and had several flats. The spare was the same size as the others, so no problem. This week I had my first flat with my 08 Expy, and the repairman pointed out it was a different size! The spare is 265-70R17 while the mounted tires are 255-70R18. Is there a reason for this, or was iit a factory mistake? I checked the manual and there is some gobbbledegook about "dissimilar sizes," including that one should not tow with a dissimilar spare mounted. I bought the Expy to tow, but should I have checked the size of the spare when I did? Anyway, it got me the 150 miles of gravel road into Goose Bay, but still---

 

Its pretty common. The overall diameter is pretty close to the same, but its a heck of a lot cheaper to put a common sized 17" tire on a steel rim than it is to match a specialty-sized 18" tire and wheel just to have as a spare.

As long as the weight rating of your spare is enough to handle your towing load, I personally wouldnt have a problem driving on it long enough to get the regular tire repared/replaced.

 

I think because 17" is the standard tire for the XLT and Eddie Bauer models. The 18 and 20" wheels are sort of upgrades. 1" difference won't kill your truck.

Another thought is there might not be room for a 18 or 20" tire under there.

 

I understand what you guys are saying and appreciate your assurances. I gather I do not have reason to gripe to the dealer when I get home, but lesson learned. Tire compatibility is a major concern when trailering in out of the way places. Next time I purchase any vehicle I will check the spare size. I doubt very much that I could have found a replacement 18" tire here in Goose Bay, Labrador, had it been more than a simple puncture, and I am a long ways from a major city. I had a blowout when driving the 04 Expy in SE Oregon and went to Burns, the only sizeable town in that part of the state, to find a replacement. None anywhere in town, and since I was just trailering through I did not care to wait several days for a delivery. Since the spare was the same size as the mounted tires (all 17s I think) I had it transferred to the blown tire's wheel and bought a cheap used "dissimilar size" to carry as spare--and that got me all the way home to Florida.
But as I said, lesson learned.

 

Good idea, don't gripe or it will show the dealer your ignorance. It states clearly in the printed and online versions of the Expedition and EL brochures that with the 18" tire ordered, you will still get a 17", full size, spare.

Because I do tow 5400 lbs, and do so in way out of the way places, I ordered 17" wheels and tires all round because of their availability and lower price. The Expy came with all season Contitrac TRs and the spare was a Contritrac SUV. Same size and specs, still an all season, but slightly different tread pattern.
One inch difference will make a difference when on one tire only, for the functioning of the ABS system/antiskid/antirollover system, and in rear diff lubrication but really shouldn't harm the vehicle over a few hundred miles until you can find the right replacement or repair. Not a good idea for a permanent installation however for the aforementioned reasons of system calibration/performance problems.

 

Your 18" tires have a diameter of 32.06 inches, and your 17" spare has a diameter of 31.61 inches, meaning the difference is .45 inch--a little over 1%. If your regular road tires wear down to about half their tread depth and the spare is new, they will be virtually identical in diameter. If your regular road tires are near the end of their life and the spare is new, the spare will be larger than the road tires.

I'm guessing that Ford is not able to source 18" steel wheels and corresponding tires as cheaply as the 17's. Given the closeness in sizes, I think you have nothing to worry about if the spare is a 265/70 and the road tires are 255/70's. You would have a larger difference in effective size if one of your road tires was 10 lbs low on air. Do keep your spare aired up.

George

 

Well said, there is much information avilable on line, especially at the fleet site. As an Engineer, I over-research everything before I purchase and then after I finally make a decision and make the purchase. Short of the Factory Shop Manuals (which I don't have yet), the owners manual, odering guide, brochures, acesssory guide, and other supplements are available free-of-charge online. I referenced all of these before test driving and felt I knew the specs of the vehicle better than the salesperson, who by the way don't like it when you correct them. I was asked more than once, are you an Engineer?