Better Ratio ?

Well it is a balance. To get best mpg you will need to run at a moderate pace. Obviously you will run some big hills now and then, so I would opt for the 3.55 because they would have you in a good rpm band for serious work while running at 65 or less. 65 would put you in the 1450 rpm area. A good area for working even rolling hills with heavy loads with an ISX. If you are wanting to stretch your legs, then 3.36 would be the choice to keep rpms from screaming while moving at a faster pace. But what you save in keeping the rpms down a little, you negate in running at a high pace. And you would have to attack a hill at a good clip to have the rpms at the right spot for a hard pull and you might actually have to drop a gear sooner than with 3.55's. Tough call. Just look at the numbers, figure how you want to run, and make a choice.

There is something to be said for keeping rpms down for fuel mileage, but that really only plays out well unless on flatter ground, light loads, and not battling 30 mph crosswinds.

 

if that is the only options then I would do the 3.55's . I would rather do 3.70's for the mountains.

I think they would pull the hills better and you would lose just a couple of mph off the top end .

I had a Volvo in 86 with a 425 cat a 13 and 3.70's and my buddy had a freightliner with a 425 cat and a 13 with 3.55's I could get to his back doors on the flat but not pass him.

In the hills I would walk around him like it was nothing . The 3.70's out pulled his 3.55 every time light or heavy .

the fuel mileage was about the same in the mountains for both but on the flats he got a couple of tenths better than me .

 

I have the 3.55 with an ISX and 13 spd but with 24.5 LP. I don't haul much weight so the low rpms and hills isn't too much of a problem, and I can get by at about 58mph in 13th in Commiefornia and have a little bit of rpm for slight hills.

If you are going with 22.5 I would go with 3.42 which I have seen on Volvos in the truck papers.

 

I guess, but I have 3.42 in hooked up to a 13 spd behind an ISX and can't stand the setup for doing anything but light loads. I run too many two lanes and hilly areas with heavy loads and 3.42's are anything but right for that work. Would have gone ahead and finally changed to 3.55, but am selling the truck this summer.