Originally Posted by
PBurns
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---
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