I have labored over this decision for quite some time and I have looked over several posts with pics and still labor over it daily. Here is my delima, I have an 09 FX4 with 18" wheels. Almost 3 mos ago, I ordered a set of moto metal 956s in 20X9. They have been on back order since that time and I being told the new arrival date should be sometime in the next two weeks. Well that has me thinking, should I wait for what is supposed to be just two more weeks and take the 20's if they come in or go ahead and switch the 18s that are in stock.

My reason for thinking about the 18s is I have been reading some of the posts on here and other forums about spare tire issues and it has got me thinking about my spare issue then. If I stay with 18s at least the rim size would be the same but I would still have to purchase a tire the correct size to match. If I go with the 20's and they come in this time, I will need to buy a 20" inch rim plus a tire. Any thoughts...

 

If you want the 20s are are willing to wait, just wait.

The spare can be a different size, it's not a big deal. I have 35s inch tires on my 95 and my spare is a 31.

As long as the tire diameter is not too off (like 10 inches) it will work fine. You have differentials so if one tire spins more because of a size difference that is ok. After all, whenever you turn in any direction they are spinning at different speeds.

 

Rim size doesn't matter, it's all about over all tired size

 

Horribly wrong, if the spare is on a rear wheel and the vehicle has a limited slip diff. You'll burn the clutches up using mismatched tires on an LS axle.

You'll also have ABS issues.

Your 95 isn't quite the same as a 2010...

 

I have 275/55/20 on the ground and 275/65/18 for a spare and there is only about three revolutions per mile different.

 

How is this horribly wrong? There is rarely a time when any given 2 wheels spin at the same rate. Unless you are going in a perfectly straight line all 4 wheels are spinning at a relatively different rate. Having 1 tire that is a slightly different size would have the same effect as driving in a circle. 1 wheel may get more power but that is how the differential is designed.

If you are locked and in 4x4, yeah that won't work.

My 05 f150 had 35s on it and I drove around with the factory spare for quite some time while 1 of the nittos was on back order. Absolutely no ill effects.

EDIT: I am not trying to start an argument. I am just going with what I know about a differential and from my experience. Seems like common sense.

 

My 2010 king ranch came with 20 inch wheels and my spare that came withthe car is an 18 inch. I checked with the dealer and he told me that is the way they come from the factory. Hope that helps.

 

personally when I get new rims and tires for my xlt I'm down sizing to 17's for a couple of reasons. One is to increase payload capacity. Another is I just like the look of more tire than rim. Just my 2cents

 

Look at it this way. If you need the spare because of a flat it will only be on the truck long enough to get somewhere to have the flat fixed. I have gone up in tire size with all four trucks I've owned and never upgraded the spare. Spare tires on cars are usually those little donuts which are a lot smaller then the OEM's. Not to mention, its not the rim size that is the issue. Its the tire size. If it really worries you that you have a smaller diameter tire then buy a cheap tire for the spare 18" rim that is the same as the tire you will have on the new 20" Moto's

 

DO NOT....drive around with two different size tires (different diameter...not rim size).....it will destroy your diff in short order.....1st...if its a limited slip it will burn up the clutches in under 20 miles.....

If its not a limited slip..it will burn up the spider gears in 50-100 miles......they're not on bearings rather a shaft which will begin to heat up and eventually destroy them....taking the rest of the bearings and gears in your diff along a few months down the road.....

 

but....that said....275/55/20's and 275/65/18's are the same diameter and can even be the same exact rev's per mile.......so they'll work fine

 

Thanks for all the great info.