So I bought a 2018 Lariat with 20" rims back in November 2017. I messed up my sidewall of my front tire (nasty pothole and rebar) and needed to get it changed. Dealership said they had to order the tire so I asked them to put on the spare. My service rep comes back and says... "Can't put on the spare, it is the wrong size tire". I thought he was joking. He said it becomes a safety issue by putting on the 18" spare when the other tires are 20". I think it is about 1" in difference in total circumference so the 18" spare would have 3" less travel on a full rotation. He said it would basically be sliding down the road.

So what can I do now? I only got the upgraded 20" due to the salesman convincing me that they were better and would look a lot nicer. I would never have bought the 20" if I knew I would have a matching spare.

The new tire came in the next day and the mechanic said there was not enough room to mount the 20" tire in the spare wheel well due to the heat shield guarding against the exhaust.

Any ideas? Will Ford replace my rims and wheels with 18" or at least go in at 50% with me?

 

So I can use the 18" spare if needed and not worry too much about it. Biggest concern is that we take the truck on vacations. If I got a flat and couldn't use the spare to get to our destination, then 2 days of our vacation would be consumed with getting the tire replaced if it couldn't be fixed...

On the ford website they say to only use the dissimilar spare minimally until a change can be had. But could I use the spare to finish off a 300 mile trip? We go to the beach.

 

what i do with different size tires is use the smaller tire up front where it will not effect the differential and not use 4 wheel drive, and use the same size tire in the rear.
this arrangement is a pain in the asterisk when you get a rear flat as you end up changing two tires, but you then do not have to worry about having different size tires on a limited slip rear differential.