Should I change diff ratio or tire/wheel size?

I have a 2006 Peterbilt 379 long hood with a 475 ISX and 18 speed trans, not sure of exact model trans. It has 3.70 diffs and currently has 11R 22.5 rubber. The cab plate says it originally came with 11R 24.5s. I can squeeze 6.2 mpg out of it running 59 mph, roughly 1350 rpm.

A local truck wrecker has a same year/model Pete with 11r 24.5s, and 3.36 rears.

The question is, am I better off swapping for the taller rears, (would like to be able to run 62-65mph at roughly same mpg) or the taller rubber? Pros/cons of each?

Thanks in advance.

 

Prairie Boy, I'm not looking to be a 75mph truck. I would just prefer not being a rolling roadblock all the time. I run 59 now, would only ever run 65 max. I'd like to be able to run the speed limit in Ontario and Quebec without losing 3/10ths mpg.

Les2, switching wheels/tires is probably easier but may actually be more expensive. I'm not sure if going to 3.36s is going too far, considering that even if I could do it without killing my fuel mileage, I very rarely run over 62mph.

 

Going to 24.5s gets me to 61mph at 1300rpm and 65 at 1400. Going to 3.36s gets me 64mph at 1300, and 59 at 1200. The truck is governed to 65 anyway (thanks Ontario and Quebec). The way that it's set up now it's turning a bit over 1500 at 65, which is too high in my opinion.

Some websites claim that the ISX actually wants to run between 1450-1550, but I would think that less rpms is better for fuel mileage, assuming that the available torque level is the same.

If I went to 3.36 and kept my 22.5s, I'd guess 62mph at 1250 or so. Honestly not sure which way to go here.

 

So using that 1350 figure going with the 24.5s might be the better bet? It would explain why the truck came with them originally. Running 1350 with 3.36s and 22.5s puts me past the 65mph speed limiter.

 

Additional thought: might it be better yet to go with the 3.36s but down to LP 22.5s? Do the diffs now, save for rubber.... really having trouble processing all this.