Seems like all tires are E-rated now..

A light read on the facts of rolling resistance from les.

https://www.schwalbetires.com/technology-faq/rolling-resistance/

 

I was one of the first people to measure bicycle tire rolling resistance (Crr) at home (rollers and a power meter). A couple things that were apparent right away were that larger tires have less rolling resistance in general, and heavy tires in the same size could have less resistance than light ones also. Compounds used and construction details were very important, yet since no one was measuring Crr, we assumed that lightness was equal to goodness for the best race tires. Durability, longevity, and puncture resistance are other considerations and though these weren't measured, everyone had a large collection of anecdotes to go by. Even though the science and equations of motion for a cyclist+bike were well developed a century ago, there was near zero attention paid to it. Everyone was obsessed with weight... probably because everyone had a scale or had access to one, and it was easy to measure.

Larger tires (wider and taller) naturally have less rolling resistance than smaller ones, and it's been discovered that lower pressure is also advantageous for lowering resistance on real roads (vs glass smooth surfaces). Pro road racers typically use 28mm tires now at ~75 psi (vs 18-23mm in the 80s, at 125 psi) and the rims are made to accommodate this size and still be aerodynamic. Another nice development is that many people are measuring Crr now, so manufacturers cannot ignore this.

Buyers of truck tires are in the stone age. No one measures Crr... so instead everyone is focused on weight... and of course durability. Weight is a poor proxy for rolling resistance. Weight is also only a minor consideration for momentum; around 2% difference worst case, comparing 40lb to 80lb tires with a lot of hard accelerations and hard braking. At steady speeds the momentum factor drops to zero.

More on topic for this thread, I'd expect E rated tires to have greater Crr than C or D, all else being equal. I'd also expect larger diameter and wider tires to have less rolling resistance, all else being equal. This doesn't necessarily translate to better MPG though, because aerodynamics dominates at high speeds, and larger diameter tires will change the gearing... which could be good or bad depending on how the ECU deals with that.

Since no one is measuring Crr of truck tires, I think the best proxy we have is the speed rating when looking at tires of the same size. Speed rating is based on temperature, and Crr (internal friction due to hysteresis) is exactly what makes the tire get hot. For instance the the 325/65r18 Territory ATs I linked earlier have a T speed rating, which is why I think they'd be good. Among tires of similar size only a few are even S speed rated; R is typical and Q isn't rare.

 

the new wildpeaks at4w come in 325/65/18 (34.6/13.2) 8 ply / D I'm going to run these vs the 35x12.5r18 which is a 12 ply / F.. which is far to stiff for our trucks. the tires are similar weight though quite heavy 75lbs

 

That 325/65 has a diagonal of 37”. Meaning, looking down on the tire from straight above, a diagonal line from the front outside corner to the rear inside corner measures 37”.
A 35 (true measurement 34.5) x 12.5 has a diagonal of 36.69”.

Your 325/65s may fit slightly worse than 35x12.5.

If no concern, carry on. But a 75lb tire is kinda insane.

 

the 35x12.5r18 in the at4w is (35.1x13) as per falken, the 325/65r18 is (34.6x13.2) so its actually not as tall and a smidge wider than the "35"
not really concerned about the weight of the tire honestly just want a better ride that E or F rated.