My opinion is that with the 6, 8. and 10 speed transmissions that are the norm now, axle ratio just isn't as critical as it was back when all we had were 3 or 4 gears to choose from. And seeing what ratios are available now (really what ratios are NOT available) I believe the engineers are in agreement.
My old 97 F-350 dually with the 7.3 had 4:10 gears. And needed them! I had around 500 or so foot/lbs of torque (chipped) and it did an amazing job pulling what ever I asked it too.
Or at least it seemed amazing. Right up until I got my current F-350! The difference is like night and day. With twice the torque and horsepower of the old engines and more gears, these newer trucks move the loads effortlessly. At least this is true here in the diesel camp.
As a matter of fact, the truck I make a living with (2021 Pete) w/Cummins X15 of the 500 hp variety with 1650 ft/lbs of torque, has 2:85 axles with a 12 speed auto shift transmission. This rig is rated at 80,000 lbs and we've had over 82K on it a couple of times. And depending on the load, we get anywhere from 7 to 10+ mpg. Our empty weight is around 38K lbs for tractor and trailer.
I know it's kinda apples and oranges comparing this to the one tons and lesser trucks that we use to tow our RV's but these newer transmissions have changed the game for sure. I'm sure the higher ratios in our pickup trucks are a result of the transmission options and I'm also convinced that the reason you can't get that 3:73 or 4:10 gearing is to preserve the manufacturers CAFE numbers.
https://www.nhtsa.gov/laws-regulatio...e-fuel-economy