Got some 2014 18" Laramie rims, now which size tire?
 

While working at the dealership in another capacity I got a great deal on a takeoff set of the 18" Laramie rims from a 2014 2500. I have wanted to move up to 18" since the newer Ford and GM trucks went to that size in 2010 or so. I like how the new rims have the center cap built in, but I will have to have the center holes bored out to 4.25" to clear my Spyntec hubs. I will find or have made a center cap for the rears.

Now that I have the rims I need to decide on the tire size to run. As for the tires, I will likely go with another set of the Michelin A/T 2. The current set has been the best set of tires I have ever run on any truck. I would also consider the Michelin M/S 2, but my truck does see dirt occasionally. I will be purchasing tires before summer as my current set is significantly dry rotted after 4 years in the AZ sun. My truck is stock height with stock size 265/70R17 tires until now.

LT265/70R17 31.7" tall, 8.1" tread width, 17/32" tread depth

Here are the available options:

LT275/55R18 - 32.1" tall, 8.4" tread width, 17/32" tread depth
LT265/70R18 - 32.7" tall, 7.7" tread width, 14.5/32" tread depth
LT275/70R18 - 33.2" tall, 8.0" tread width, 14.5/32" tread depth

* all measurements from Michelin's spec sheet

I believe there is an unloaded fuel economy advantage to going with a slightly taller tire, it effectively reduces the final drive ratio like putting in a numerically lower ring and pinion. If you go too big or too wide it will cost fuel economy. I have been using the truck for more highway trips recently. The final ratio differences with different tires:

LT275/55R18 - 32.1" tall, 3.67 equivalent ratio
LT265/70R18 - 32.7" tall, 3.62 equivalent ratio
LT275/70R18 - 33.2" tall, 3.55 equivalent ratio

One other factor is speedometer calibration. I am not running a tuner. The Dodge dealership software only allows for preset tire sizes, in the range I am looking at the only options are 265/70R17 and 285/70R17. The 285/70R17 at 32.7" is the same as the 265/70R17, my speedo would be dead on. Without correction and the 275/65R18 the speedo would read 1.2 mph low at 75 mph. The differences are very minor, so not that much of a consideration.

At this point I am leaning towards the 275/65R18 or the 265/70R18. The 275/70R18 are not on the short list because I think they are too big to keep my economy. On the 275/65R18 I like the 32.1" diameter and the 17/32" tread depth, but I am not thrilled with the extra width as it will not help economy. The 265/70R18 are narrower than what I have now in the tread and have less tread at 14.5/32". The 32.7" height may still be ok for fuel economy.

Thoughts?

285-70 or 75-18 AT2's. 55 or 65 series sidewalls aren't going to help in the ride dept.